This fixes https://github.com/postmarketOS/binary-package-repo/issues/1
GCC generates hardlinks between files `A` and `B` in its `make install` step. The problem is, that `tar` randomly packages `A` as full binary, and links `B` to `A`, or the other way around! I was able to reproduce this issue consistently when re-building `gcc-aarch64` on Travis CI (interestingly, this did not appear for `gcc-armhf`).
The fix is, to delete `B` and create a symlink `B` that points to `A` instead.
* pmb/challenge/apk.py had to be renamed to pmb/challenge/apk_file.py,
so the "internal" functions of that file could be accessed, while
still providing the short notation pmb.challenge.apk().
* zap asks for each buildroot_* chroot, if you want to remove it, not
only for the one with the device arch
* add new pmb.chroot.tempfolder() function, that creates a temporary
folder, that belongs to "user" and deletes it, if it already exists.
this function gets used in a few challenge testcases.
