spl: Kconfig: Replace CONFIG_SPL_FAT_SUPPORT with CONFIG_SPL_FS_FAT

Replace CONFIG_SPL_FAT_SUPPORT with CONFIG_SPL_FS_FAT so
obj-$(CONFIG_$(SPL_)FS_FAT) can be used to control the build in both
SPL and U-Boot.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/fs/Makefile b/fs/Makefile
index f21cd23..bebea6a 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -6,7 +6,7 @@
 
 ifdef CONFIG_SPL_BUILD
 obj-$(CONFIG_FS_LOADER) += fs.o
-obj-$(CONFIG_SPL_FAT_SUPPORT) += fat/
+obj-$(CONFIG_SPL_FS_FAT) += fat/
 obj-$(CONFIG_SPL_EXT_SUPPORT) += ext4/
 else
 obj-y				+= fs.o