* New commandline parameter --mirror-pmOS, where the binary repository
URL for postmarketOS can be specified (empty by default as of now,
this will be filled with the real URL once the repo works)
* Do not build packages, when they are in the binary repository and
the version of the package in the binary repository is up-to-date.
* Add a testcase for pmb.build.is_necessary().
...even if the pkgver and pkgrel have *not* changed. This should
make development much more intuitive. The detection works by looking
at the last modified timestamps, just like `make` does it.
* The supported architectures are inside the config now
* Symlinks get created for that list of supported architectures now.
* During initialization, the architecture from the selected device
gets checked against the list of supported architectures. When
it is not included, a meaningful exception gets raised.
* the aportgen and (cross-compiler) build tests make use of the
new variable now (they had armhf and aarch64 hardcoded previously).
