mtd: block2mtd: wait until block devices are presented

Ensures that block2mtd is triggered after the block devices are enumerated
at boot time.
This issue is seen on BCM2835 (Raspberry Pi) systems when mounting JFFS2
block2mtd filesystems, probably because of the delay on enumerating a USB
MMC card reader.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Rodrigo Freire <rfreire@redhat.com>
Signed-off-by: Herton Krzesinski <herton@redhat.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>