makechrootpkg: Add option to build in temp chroot

Add option -T to build in a temporary chroot. This apply to any kind of
filesytem and allow to easily parrallelize builds.

This patch also simplify how $default_copy and $copy are defined.

Signed-off-by: Sébastien Luttringer <seblu@seblu.net>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>