Add option to use Ada code in ramstage

If selected, libgnat will be linked into ramstage. And, to support Ada
package intializations, we have to call ramstage_adainit().

Change-Id: I11417db21f16bf3007739a097d63fd592344bce3
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/16944
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/Kconfig

@@ -1243,3 +1243,8 @@ config CHECKLIST_DATA_FILE_LOCATION
 	  symbols contained only in <stage>_complete.dat will be flagged as
 	  required and not implemented if a weak implementation is found in the
 	  resulting image.
+
+config RAMSTAGE_ADA
+	def_bool n
+	help
+	  Selected by features that use Ada code in ramstage.