cmd_extX: use common get_device_and_partition function

cmd_extX: use common get_device_and_partition function

Convert ext2/4 load, ls, and write functions to use common device and
partition parsing function. With the common function "dev:part" can come
from the environment and a '-' can be used in that case.

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