init: Make a better detection of root device

Grep /chroot can return many other sub mounted directories like /boot
etc..
init: Deleting mtab for Redhat OS

mtab shall not be kept over deploiement unless some FS will be
considered as mounted at boot time.
init: Let an option to get a global BOOT_ARG

BOOT_ARG could be defined to get more common boot arguments, so let an
option to define it before.