Convert CONFIG_SPL_FAT_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/draco_defconfig b/configs/draco_defconfig
index 80f2cd8..e00e166 100644
--- a/configs/draco_defconfig
+++ b/configs/draco_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_DRACO=y
 CONFIG_SYS_MALLOC_F_LEN=0x2000
+CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-draco"
 CONFIG_BOOTDELAY=3
 CONFIG_SPL=y