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/ti814x_evm_defconfig b/configs/ti814x_evm_defconfig
index 934136c..2ab3fad 100644
--- a/configs/ti814x_evm_defconfig
+++ b/configs/ti814x_evm_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_TI814X_EVM=y
+CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_BOOTDELAY=1
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y