Kconfig: fix SPL_FIT dependency

SPL_FIT obviously requires libfdt in SPL, so let Kconfig express that by
selecting SPL_OF_LIBFDT.
Also make the actual options that users want (SPL signature and SPL FIT
loading) visible in the menu and let them select the SPL_FIT as a
requirement.
Also remove the now redundant SPL_OF_LIBFDT from those Kconfigs that had
it in for the SPL FIT loading feature.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
[Remove change from configs/evb-rk3399_defconfig]
Signed-off-by: Jagan Teki <jagan@openedev.com>
diff --git a/Kconfig b/Kconfig
index 1cf990d..deacec4 100644
--- a/Kconfig
+++ b/Kconfig
@@ -208,15 +208,17 @@
 config SPL_FIT
 	bool "Support Flattened Image Tree within SPL"
 	depends on SPL
+	select SPL_OF_LIBFDT
 
 config SPL_FIT_SIGNATURE
 	bool "Enable signature verification of FIT firmware within SPL"
-	depends on SPL_FIT
 	depends on SPL_DM
+	select SPL_FIT
 	select SPL_RSA
 
 config SPL_LOAD_FIT
 	bool "Enable SPL loading U-Boot as a FIT"
+	select SPL_FIT
 	help
 	  Normally with the SPL framework a legacy image is generated as part
 	  of the build. This contains U-Boot along with information as to