moving to initramfs unifies disk allocate on different platforms.
use fallocate instead of dd to speed up the disk allocation.
By default, mkfs.ext4 has -E discard option which discards the blocks
at the mkfs time, also speed up the initialization time.
On some platforms after the initramfs init scripts have mounted the root device its
block device disapear from /dev. The union-mount script therefore can't mount the
root device over the aufs. If this case happen, issue a rescan of the devices to
repopulate the /dev filesystem.
