spl: spl_mmc: Clearer structure in spl_mmc_load_image and cosmetics

This refactors spl_mmc_load_image to use a switch/case structure and easier
to understand spl_start_uboot checks. This also introduces some more automatic
fallback on the next mmc boot mode as long as it keeps failing.

Lines that go beyond 80 chars are also reduced by reducing the number of tabs.
Debug and error strings are refctored to match a common style.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
[trini: Fix a thinko in the SPL_EXT_SUPPORT + SPL_OS conversion, make part be
 __maybe_unused]
Signed-off-by: Tom Rini <trini@konsulko.com>