bootm: Add a function to check overlap

bootm: Add a function to check overlap

Move this code into a function to reduce code size and make it easier
to understand. Drop the unnecessary 0x to help a little with code size.

Use this in bootm_find_images()

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