soong: rework soong_config for Pie

Change-Id: I3c5001f02975e5487ba66a02217f1d24d2021272
diff --git a/build/soong/soong_config.mk b/build/soong/soong_config.mk
index eed8e09..46d8c29 100644
--- a/build/soong/soong_config.mk
+++ b/build/soong/soong_config.mk
@@ -1,7 +1,8 @@
-omnirom_soong:
-	$(hide) mkdir -p $(dir $@)
-	$(hide) (\
-	echo '{'; \
-	echo '"Omnirom": {'; \
-	echo '},'; \
-	echo '') > $(SOONG_VARIABLES_TMP)
+_contents := $(_contents)    "Omnirom":{$(newline)
+
+# See build/core/soong_config.mk for the add_json_* functions you can use here.
+
+# This causes the build system to strip out the last comma in our nested struct, to keep the JSON valid.
+_contents := $(_contents)__SV_END
+
+_contents := $(_contents)    },$(newline)