vendorcode/eltan/Kconfig: Hide the Kconfig options when lacking support

The vendorcode/eltan mboot and verified boot options only build if a
few other Kconfig options are defined.

Change-Id: Ie333d2fbf294e23ec01df06ee551e2d09541c744
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35954
Reviewed-by: Wim Vervoorn
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/facebook/fbg1701/Kconfig

@@ -30,6 +30,7 @@ config BOARD_SPECIFIC_OPTIONS
 	select DISABLE_HPET
 	select INTEL_GMA_HAVE_VBT
 	select GENERIC_SPD_BIN
+	select USE_VENDORCODE_ELTAN
 
 config ONBOARD_SAMSUNG_MEM
 	bool "Onboard memory manufacturer Samsung"

src/vendorcode/eltan/Kconfig

@@ -13,5 +13,10 @@
 ## GNU General Public License for more details.
 ##
 
+config USE_VENDORCODE_ELTAN
+	bool
+
+if USE_VENDORCODE_ELTAN
 source src/vendorcode/eltan/security/mboot/Kconfig
 source src/vendorcode/eltan/security/verified_boot/Kconfig
+endif