We used to use struct image_data as the central data structure for bootm and
also as the input data structure. This makes it unclear which of the fields
are actually input data. This patch creates a struct bootm_data which is
exclusively used for input data to make usage clearer.

Also it moves the dispatching of multifile uImage pathnames to the core bootm
code so that the core code gets more flexible and the command code simpler.

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