init: Rsync /post_rsync instead of many copies

All .configure files needs to adjust some configuration on the operating
system that will be deployed.

Before the patch, the .configure where putting files in / and init was
copying them by hand to the chroot after the rsync got done.

That is not scalable since adding a new file requires to patch init.

This patch creates a /post_rsync directory where .configure files can
put the file they need like if it was a chroot. Once the operating
system got installed, this directory will be rsynced over the chroot.

i.e: If you need to override the /etc/hosts file, your .configure will
have to write the file in /post_rsync/etc/hosts.

Some directories are created by default but it your configure needs
more, feel free to create them under /post_rsync/.

This patch will reduce the number of hacks inside init and make
.configure changes less intrusive.