disk/part: introduce get_device_and_partition

All block device related commands (scsiboot, fatload, ext2ls, etc.) have
simliar duplicated device and partition parsing and selection code. This
adds a common function to replace various implementations.

The new function has an enhancement over current versions. If no device
or partition is specified on the command line, the bootdevice env variable
will be used (scsiboot does this).

Signed-off-by: Rob Herring <rob.herring@calxeda.com>