bootm: Adjust the parameters of bootm_find_images()

bootm: Adjust the parameters of bootm_find_images()

Rather than passing it all the command-line args, pass in the pieces
that it needs. These are the image address, the ramdisk address/name
and the FDT address/name.

Ultimately this will allow usage of this function without being called
from the command line.

Move the function comment to the header file and tidy it a little.

Signed-off-by: Simon Glass <sjg@chromium.org>