Add flag for including a mirrorlist in the pacman.conf.

The flag only has effect at chroot creation time, of course.  By default it's
on.  When off, only the mirror at kernel.org will be used.

This turns out to be useful if running makeahpkg on an Arch derivative distro.

Signed-off-by: Magnus Therning <magnus@therning.org>
Merge branch 'master' of https://github.com/archhaskell/habs