Nokia RX-51: Load bootmenu also from uSD card

By default bootmenu is loaded only from eMMC. After this change U-Boot
first tries to load bootmenu from uSD card and if it fails then fallback to
eMMC. People want to boot alternative OS from removable uSD without need to
modify eMMC content. So this backward compatible change allows it.

Part of this change is also optimization of trymmc* macros so they can be
used in preboot macro for loading bootmenu.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210618132704.32066-2-pali@kernel.org