config_fallbacks: add additional fallbacks for fat filesystem

Add fallbacks needed to keep all boards building
while they are migrated to use Kconfig symbols
instead of defines in <board>_config.h files for
FAT filesystem.

These should eventually go away once Kconfig select
or imply statements are put in place and duplicated
defines in <board>_config.h removed.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
[trini: Update logic since CMD_FAT / CONFIG_SPL_FAT_SUPPORT are
        selecting FS_FAT]
Signed-off-by: Tom Rini <trini@konsulko.com>