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 <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Ian Campbell <ijc@hellion.org.uk>
Cc: Hans De Goede <hdegoede@redhat.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Jagan Teki <jteki@openedev.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>