fs: fat: add kbuild configuration support

Add Kconfig symbols for various configurations
supported by FAT filesystem support code.

CONFIG_SUPPORT_VFAT has been left out since its
force enabled in include/fat.h and probably
should get removed at some point.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
[trini: add select FS_FAT for CMD_FAT and SPL_FAT_SUPPORT]
Signed-off-by: Tom Rini <trini@konsulko.com>