build: soong: put omnirom variables in the "Omnirom" struct

Change-Id: I9ed55cea0c55f0078f8f97ad4037b16ed11c0156
diff --git a/build/soong/soong_config.mk b/build/soong/soong_config.mk
index cffe4c2..eed8e09 100644
--- a/build/soong/soong_config.mk
+++ b/build/soong/soong_config.mk
@@ -2,4 +2,6 @@
 	$(hide) mkdir -p $(dir $@)
 	$(hide) (\
 	echo '{'; \
+	echo '"Omnirom": {'; \
+	echo '},'; \
 	echo '') > $(SOONG_VARIABLES_TMP)