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/cm_t43_defconfig b/configs/cm_t43_defconfig
index f52ee36..6c5ff9f 100644
--- a/configs/cm_t43_defconfig
+++ b/configs/cm_t43_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_CM_T43=y
 CONFIG_SPL_EXT_SUPPORT=y
+CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_HUSH_PARSER=y