uboot-imx

nand commands: make only "dump" repeatable.

The dump command is made to increment its address on repeat,
as md does.  Other commands do not make sense to issue repeatedly,
and can be irritating when it happens accidentally, so don't.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Tested-by: Ben Gardiner <bengardiner@nanometrics.ca>