bootm: refactor image detection and os load steps

Created a bootm_start() that handles the parsing and detection of all
the images that will be used by the bootm command (OS, ramdisk, fdt).
As part of this we now tract all the relevant image offsets in the
bootm_headers_t struct. This will allow us to have all the needed
state for future sub-commands and lets us reduce a bit of arch
specific code on SPARC.

Created a bootm_load_os() that deals with decompression and loading
the OS image.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>