sandbox: Update os_find_u_boot() to find the .img file

At present this function can only locate the u-boot ELF file. For SPL it
is handy to be able to locate u-boot.img since this is what would normally
be loaded by SPL.

Add another argument to allow this to be selected.

While we are here, update the function to load SPL when running in TPL,
since that is the next stage.

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