bootstd: support scanning a single partition

The "bootflow" command currently doesn't support scanning a single
partition. This is inconvenient in setups with multiple bootable
partitions within a single disk, but only one is desired.

Support scanning a single disk partition. Specifically, support the
syntax:
	bootflow scan mmc1:4
which scans only mmc device 1, partition 4.

Signed-off-by: Nam Cao <namcao@linutronix.de>