Tooling changes:

* Make `stable` the default branch.
* Strip binaries by default.
* Use `systemctl is-enabled` instead of `systemctl list-unit-files` to
see whether a unit is enabled or not.

Installer changes:

* Improve migration path from remarkable\_entware regarding the location
of the `opt.mount` configuration file.
* Keep existing branch configuration if an install of Toltec already
exists.

Updated packages (not listing the packages that were only updated
because of the strip changes):

* chessmarkable 0.5.1-1 → 0.6.0-2
* koreader 2020.10-2 → 2020.12-1
* oxide 2.0.1\~beta-3 → 2.0.3\~beta-1
* rm2fb 1.0.0-3 → 1.0.0-5
* remux 0.1.4-1 → 0.1.5-1

New packages:

* keywriter 0.1.0-1
* restream 0.0.0-1
* rm2-suspend-fix 0.0.0-1
* genie 0.1.0-1