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/pcm051_rev3_defconfig b/configs/pcm051_rev3_defconfig
index 76b94c4..f449f58 100644
--- a/configs/pcm051_rev3_defconfig
+++ b/configs/pcm051_rev3_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_PCM051=y
 CONFIG_SPL_EXT_SUPPORT=y
+CONFIG_SPL_FAT_SUPPORT=y
 CONFIG_SPL_ENV_SUPPORT=y
 CONFIG_SYS_EXTRA_OPTIONS="REV3"
 CONFIG_VERSION_VARIABLE=y