initramfs-framework: move storage mounts to actual rootfs

Operations such as mkfs fail on devices that are not
switched to the actual rootfs before switch_root is
called. The kernel interprets these devices as still
being used even after unmounting and errors such as
below are seen when the target is fully booted

root@v1000:~# umount /dev/sdb1
root@v1000:~# mkfs.ext4 /dev/sdb1
mke2fs 1.43.8 (1-Jan-2018)
/dev/sdb1 contains a ext4 file system
        last mounted on Wed Nov 28 07:33:54 2018
Proceed anyway? (y,N) y
/dev/sdb1 is apparently in use by the system; will not make a filesystem here!

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Muhammad Hamza <muhammad_hamza@mentor.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>