ubiformat: use default values for ubi_ver and image_seq ...

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

commands: ubiformat: move code to common/ ...

This is the final step to separate the ubiformat code from the
command. With this the ubiformat code gains a C API.

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