Alpine ships `mkbootimg` with the `android-tools` package now. This
conflicts with the `mkbootimg` fork from osm0sis (see #441).

Changes:
* Rename `mkbootimg` to `mkbootimg-osm0sis` (aport and binary name)
* `mkbootimg-osm0sis`: provides `mkbootimg` now (so we don't need to
  change all the device aports), update version to 2018.05.10
* Adjust our `mkinitfs` script to call `mkbootimg-osm0sis`
* Better aport description
* pkgrel_bump testcase: don't fail on deleted aport