commands: remove struct command pointer from commands ...

This is unused in all commands and thus can be removed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

add uimage command ...

The uimage command superseeds the iminfo command. Unlike
the iminfo command the uimage command can also be used
to verify the data crc on demand and to extract uImages
to files.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>