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/board/ti/common/Kconfig b/board/ti/common/Kconfig
index 5173e6b..c4c0b4f 100644
--- a/board/ti/common/Kconfig
+++ b/board/ti/common/Kconfig
@@ -9,3 +9,6 @@
 
 config SPL_EXT_SUPPORT
 	default y
+
+config SPL_FAT_SUPPORT
+	default y