* 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.
