Fixes:

• [buildrootfs] Now uses a provided pacstrap script, instead of relying on manjaro-arm-base's basestrap. (oversight from last release)
• [buildarmpkg] Now also prunes package cache, if the build fails. Before it would not unmount the package cache because the prune function was not run.
• [buildarmimg] Now uses the functions of pacman-mirrorlist in the chroot to generate a mirrorlist, but not ranking them. Shaves of a few minutes of image build time.
• [buildarmimg][buildarmpkg] Change branch names accepted by -b argument. Not only accepts stable, testing and unstable to be in line with Manjaro x86 branch names.