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/xilinx_zynqmp_zc1751_xm016_dc2_defconfig b/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
index 07faafa..bdb6377 100644
--- a/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
+++ b/configs/xilinx_zynqmp_zc1751_xm016_dc2_defconfig
@@ -9,6 +9,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_SPL=y
+# CONFIG_SPL_FAT_SUPPORT is not set
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="ZynqMP> "