I've replaced all instances in the code of `os.path.abspath`
with `os.path.realpath`, as this does the same as `abspath`
plus resolving symlinks.
See also: https://stackoverflow.com/a/40311142
* 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.
