spl: change return values of spl_*_load_image()

spl: change return values of spl_*_load_image()

Make spl_*_load_image() functions return a value instead of
hanging if a problem is encountered. This enables main spl code
to make the decision whether to hang or not, thus preparing
it to support alternative boot devices.

Some boot devices (namely nand and spi) do not hang on error.
Instead, they return normally and SPL proceeds to boot the
contents of the load address. This is considered a bug and
is rectified by hanging on error for these devices as well.

Signed-off-by: Nikita Kiryanov <[email protected]>
Cc: Igor Grinberg <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Ian Campbell <[email protected]>
Cc: Hans De Goede <[email protected]>
Cc: Albert Aribaud <[email protected]>
Cc: Jagan Teki <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Reviewed-by: Simon Glass <[email protected]>