Combine bootm_find_<thing> functions together

bootm_find_ramdisk_fdt() renamed to bootm_find_images() for readability.

The function bootm_find_ramdisk_fdt() appears to be a simple wrapper for
bootm_find_ramdisk(), bootm_find_fdt(), and now bootm_find_loadables().
I didn't see any other callers entering a bootm_find<thing>, so removing
the wrapper, and condensing these together hopefully makes the code a
little simpler.

Signed-off-by: Karl Apsite <Karl.Apsite@dornerworks.com>
Reviewed-by: Simon Glass <sjg@chromium.org>