functions: Reset resolv.conf when building targets

The initial resolv.conf integrated in base could have been made on a
particular network (i.e home network) while building a new target could
have been done on another network (i.e office network).

In such case, as we do rsync the chroot, all network related commands
like apt & co will fail.

This patch insure that after the rsync, we do copy the current
resolv.conf to gain the proper DNS services.