Simplify chroot setup by dropping build_config.groups

Simplify chroot setup by dropping build_config.groups

Use chroot groups of the result instead of going through
build_config.groups. Make sure res.chroot is sorted independent of
proj/chroot.

Previously we did a complicated dance adding all required groups to a
build_config dict and then iterated over the entire chroot table. The
order in which chroot groups were untared did depend on the order of
appearance in proj/chroot.

Signed-off-by: Tobias Ulmer <tu@emlix.com>