doc: Add documentation for U-Boot integration [v2]

Cover the drivers, scripts, pytest and the Gitlab information.

Note: This implementation is the result of working through what is
needed in Labgrid to support U-Boot labs. I did initially file quite
a few issues[1] but there has not been a lot of traction and I got
feedback that I have overwhelmed people with too many. So I have
stopped filing issues on the things I hit along the way. I have carried
on with the implementation in the hope that this can be a better basis
for discussion.

Note that all of these patches are work-in-progress, but it is becoming
more stable.

Feedback may still change the implementation and documentation but the
time for wholesale changes of approach has passed.

Cover-letter:
U-Boot integration
END

Changes in v6:
- Fix reference to crossbar in _ub-bisect-try and ub-smoke
- Add -x option to specify the extra board directory
- Add comments to the example lg-env
- Define USE_LABGRID to tell u-boot-hooks to use labgrid hooks
- Add a few more comments to ub-pyt
- Show the build path with ub-pyt -v
- Add --allow-unmatched to ub-smoke to avoid QEMU error
- Add a new section on compatibility
- Add a new section on script usage

Changes in v5:
- Rebase on latest grpc branch
- Fix pylint errors and warnings

Changes in v4:
- Support for Beagleplay, which needs files from two separate U-Boot
  builds
- Support for a 'recovery' button needed to boot the image
- Tidy up the internal-console support
- Fix pytest behaviour with an unpatched U-Boot (that doesn't have lab
  mode)

Changes in v3:
- Rebase on top of grpc branch
- Don't mess with terminal setting unless stdin is a terminal
- Don't show an error if there are no resources when auto-acquiring
- Support QEMU in UBootWriter

Some changes in v2:
- Rationalise the flags for the U-Boot scripts
- Support tracing with em100
- Support an internal terminal instead of microcom
- Add a -D flag for debugging
- Support send-only boards
- Add a way to build the U-Boot config
- Add a control for buildman's process-limit
- allow the build-dir to be specified in a variable
- add documentation about U-Boot-pytest integration
- add source_dir and config_file to UBootProviderDriver
- add an internal terminal
- expand the U-Boot scripts
- significantly improve the U-Boot-pytest integration

The approximate diffstat is:
 contrib/sync-places.py                |  23 +-
 contrib/u-boot/.gitignore             |   1 +
 contrib/u-boot/_ub-bisect-try         |  47 ++
 contrib/u-boot/conftest.py            |  21 +
 contrib/u-boot/get_args.sh            | 128 +++++
 contrib/u-boot/index.rst              | 232 +++++++++
 contrib/u-boot/lg-client              |  11 +
 contrib/u-boot/lg-env                 |  10 +
 contrib/u-boot/test_smoke.py          |   3 +
 contrib/u-boot/ub-bisect              |  44 ++
 contrib/u-boot/ub-cli                 |  39 ++
 contrib/u-boot/ub-int                 |  41 ++
 contrib/u-boot/ub-pyt                 |  69 +++
 contrib/u-boot/ub-smoke               |  48 ++
 doc/configuration.rst                 | 611 ++++++++++++++++++++++-
 doc/usage.rst                         | 288 +++++++++++
 labgrid/driver/__init__.py            |  10 +-
 labgrid/driver/common.py              |  11 +
 labgrid/driver/consoleexpectmixin.py  |   7 +
 labgrid/driver/powerdriver.py         |  29 ++
 labgrid/driver/qemudriver.py          |  84 ++--
 labgrid/driver/recoverydriver.py      |  25 +
 labgrid/driver/resetdriver.py         |   7 +
 labgrid/driver/servodriver.py         | 157 ++++++
 labgrid/driver/sfemulatordriver.py    | 102 ++++
 labgrid/driver/ubootdriver.py         |  27 +-
 labgrid/driver/ubootproviderdriver.py | 323 ++++++++++++
 labgrid/driver/ubootwriterdriver.py   | 160 ++++++
 labgrid/driver/usbhidrelay.py         |   7 +-
 labgrid/driver/usbloader.py           | 170 ++++++-
 labgrid/driver/usbstoragedriver.py    |  18 +-
 labgrid/factory.py                    |   4 +-
 labgrid/protocol/__init__.py          |   1 +
 labgrid/protocol/bootstrapprotocol.py |   8 +-
 labgrid/protocol/recoveryprotocol.py  |  14 +
 labgrid/protocol/resetprotocol.py     |  12 +
 labgrid/pytestplugin/fixtures.py      |  21 +-
 labgrid/pytestplugin/hooks.py         |   8 +
 labgrid/remote/client.py              | 351 ++++++++-----
 labgrid/remote/config.py              |   7 +-
 labgrid/remote/exporter.py            | 122 ++++-
 labgrid/resource/__init__.py          |   6 +
 labgrid/resource/remote.py            |  24 +
 labgrid/resource/servo.py             | 485 ++++++++++++++++++
 labgrid/resource/sfemulator.py        |  33 ++
 labgrid/resource/suggest.py           |   6 +
 labgrid/resource/udev.py              |  33 ++
 labgrid/strategy/ubootstrategy.py     | 151 +++++-
 labgrid/target.py                     | 120 ++++-
 labgrid/util/helper.py                | 220 ++++----
 labgrid/util/ssh.py                   |   3 +-
 labgrid/util/term.py                  | 184 +++++++
 labgrid/var_dict.py                   |   8 +
 man/labgrid-client.1                  |   6 +
 man/labgrid-client.rst                |   4 +
 man/labgrid-device-config.5           |   4 +
 56 files changed, 4267 insertions(+), 321 deletions(-)

[1] https://github.com/labgrid-project/labgrid/issues/created_by/sjg20

Signed-off-by: Simon Glass <sjg@chromium.org>

doc/configuration.rst

@@ -2021,6 +2021,8 @@ Arguments:
   - login_timeout (int, default=60): timeout for password/login prompt detection
   - for other arguments, see `UBootDriver`_
 
+.. _UBootProviderInfo:
+
 UBootProviderDriver
 ~~~~~~~~~~~~~~~~~~~
 
@@ -2064,6 +2066,11 @@ Variables:
     specified in the environment file
   - do-clean (str): If set to "1" this cleans the build before starting,
     otherwise it does an incremental build
+  - build-dir (str): If set, this is used as the build directory for U-Boot
+  - process-limit (int): Limits the number of buildman processes which can
+    be running jobs at once. Set this to 1 to avoid over-taxing your
+    CPU. Buildman does its own multithreading, so each process will use
+    all available CPUs anyway.
 
 Environment variables:
   - U_BOOT_BUILD_DIR (str): If present, this is used as the build directory for
@@ -2115,6 +2122,8 @@ Tools:
    tools:
      buildman: "buildman.stable"
 
+.. _UBootWriterInfo:
+
 UBootWriterDriver
 ~~~~~~~~~~~~~~~~~
 
@@ -3875,6 +3884,8 @@ the "shell" state:
 This command would transition directly into a Linux shell and
 activate the `ShellDriver`_.
 
+.. _UBootStrategyInfo:
+
 UBootStrategy
 ~~~~~~~~~~~~~
 A :any:`UBootStrategy` has five states: