spl: Migrate SYS_SATA_FAT_BOOT_PARTITION to Kconfig

This moves SYS_SATA_FAT_BOOT_PARTITION to Kconfig and enforces the
current default via Kconfig rather than C code.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>