Move build_sysimg docstring

This moves the docs for `build_sysimg` inline in `contrib/build_sysimg.jl`.
Adds an additional condition to the `build_sysimg.jl` file so that we
can safely include the file in `genstdlib.jl` without a system image being
built unintentionally.

contrib/build_sysimg.jl

@@ -12,6 +12,18 @@ function default_sysimg_path(debug=false)
     end
 end
 
+"""
+    build_sysimg(sysimg_path=default_sysimg_path, cpu_target="native", userimg_path=nothing; force=false)
+
+Rebuild the system image. Store it in `sysimg_path`, which defaults to a file named `sys.ji`
+that sits in the same folder as `libjulia.{so,dylib}`, except on Windows where it defaults
+to `JULIA_HOME/../lib/julia/sys.ji`.  Use the cpu instruction set given by `cpu_target`.
+Valid CPU targets are the same as for the `-C` option to `julia`, or the `-march` option to
+`gcc`.  Defaults to `native`, which means to use all CPU instructions available on the
+current processor. Include the user image file given by `userimg_path`, which should contain
+directives such as `using MyPackage` to include that package in the new system image. New
+system image will not replace an older image unless `force` is set to true.
+"""
 function build_sysimg(sysimg_path=nothing, cpu_target="native", userimg_path=nothing; force=false, debug=false)
     if sysimg_path == nothing
         sysimg_path = default_sysimg_path(debug)
@@ -157,7 +169,10 @@ end
 
 # When running this file as a script, try to do so with default values.  If arguments are passed
 # in, use them as the arguments to build_sysimg above
-if !isinteractive()
+#
+# Also check whether we are running `genstdlib.jl`, in which case we don't want to build a
+# system image and instead only need `build_sysimg`'s docstring to be available.
+if !isdefined(Main, :GenStdLib) && !isinteractive()
     if length(ARGS) > 5 || ("--help" in ARGS || "-h" in ARGS)
         println("Usage: build_sysimg.jl <sysimg_path> <cpu_target> <usrimg_path.jl> [--force] [--debug] [--help]")
         println("   <sysimg_path>    is an absolute, extensionless path to store the system image at")

doc/devdocs/sysimg.rst

@@ -26,10 +26,9 @@ This will include a ``build_sysimg()`` function:
 
 .. function:: build_sysimg(sysimg_path=default_sysimg_path, cpu_target="native", userimg_path=nothing; force=false)
 
-   Rebuild the system image. Store it in ``sysimg_path``, which defaults to a file named ``sys.ji`` that sits in the same folder as ``libjulia.{so,dylib}``, except on Windows where it defaults to ``JULIA_HOME/../lib/julia/sys.ji``.
-   Use the cpu instruction set given by ``cpu_target``.  Valid CPU targets are the same as for the ``-C`` option to ``julia``, or the ``-march`` option to ``gcc``.  Defaults to ``native``, which means to use all CPU instructions available on the current processor.
-   Include the user image file given by ``userimg_path``, which should contain directives such as ``using MyPackage`` to include that package in the new system image.
-   New system image will not replace an older image unless ``force`` is set to true.
+   .. Docstring generated from Julia source
+
+   Rebuild the system image. Store it in ``sysimg_path``\ , which defaults to a file named ``sys.ji`` that sits in the same folder as ``libjulia.{so,dylib}``\ , except on Windows where it defaults to ``JULIA_HOME/../lib/julia/sys.ji``\ .  Use the cpu instruction set given by ``cpu_target``\ . Valid CPU targets are the same as for the ``-C`` option to ``julia``\ , or the ``-march`` option to ``gcc``\ .  Defaults to ``native``\ , which means to use all CPU instructions available on the current processor. Include the user image file given by ``userimg_path``\ , which should contain directives such as ``using MyPackage`` to include that package in the new system image. New system image will not replace an older image unless ``force`` is set to true.
 
 Note that this file can also be run as a script itself, with command line arguments taking the place of arguments passed to the ``build_sysimg`` function.  For example, to build a system image in ``/tmp/sys.{so,dll,dylib}``, with the ``core2`` CPU instruction set, a user image of ``~/userimg.jl`` and ``force`` set to ``true``, one would execute:
 ::

doc/genstdlib.jl

@@ -200,4 +200,8 @@ validdocstr(other) = false
 
 end
 
+# The docstring for `build_sysimg` is defined is this file and included within the
+# `devdocs/sysimg.rst` file, so we include it here to make it visible to the docsystem.
+include(joinpath("..", "contrib", "build_sysimg.jl"))
+
 GenStdLib.translate(["manual", "stdlib", "devdocs"])