Take advantage of live-build 3.x exposing the config tree in the chroot, and...

Take advantage of live-build 3.x exposing the config tree in the chroot, and of its environment.chroot feature.

Cleanup custom stuff accordingly.

Note that we keep using /usr/share/amnesia/build/variables for POTFILES_DOT_IN:
live-build does not support variables whose value contains spaces, in
environment.chroot.