bootm: Simplify Android ramdisk addr in bootm_find_images()

bootm: Simplify Android ramdisk addr in bootm_find_images()

The Android mechanism uses the loadaddr envrionment-variable to get the
load address, if none is provided. This is equivalent to
image_load_addr so use that instead, converting it to a string as
needed.

This change will permit passing img_addr to this function, in a future
change.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>