When blspec_scan_cdev finds an UBI device then attach it. This
allows us to boot blspec entries found on the UBIFS images inside
UBI volumes by pointing to the corresponding mtd partition.
With this we can do 'boot nand0.root' or even 'boot=nand0' on the
commandline.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>