In A/B schemas, it is possible to have multiple rootfs.
Thus, it is necessary to let the B kernel preinitialize either the A
rootfs or the B rootfs failing to mount the A rootfs.
Ideally, we should also try to switch to the B rootfs if we cannot start
up the init on the A rootfs, but that's a FUTUREWORK.
Signed-off-by: Raito Bezarius <masterancpp@gmail.com>