This is the preparation part of rear before starting the rescue build phase.

Please note that you should not put any scripts that reference 
$ROOTFS_DIR or $CONFIG_DIR/recovery into this section. Scripts for $ROOTFS_DIR
go to rescue/ and for $CONFIG_DIR/recovery go to dr/ respectively

That way we keep only the real prep stuff here in the prep section. This is 
important because some workflows (e.g. mkbackuponly) skip the dr and rescue
sections but include the prep section so that we shouldn't do stuff in the 
prep section like creating the ROOTFS_DIR or wiping $CONFIG_DIR/recovery since
not ALL workflows require these things.
