Default USE_SOONG to true

Change-Id: Iad88aed4060b84aef2381f6300f16bad14f3a6ca
diff --git a/core/main.mk b/core/main.mk
index c455320..5834cd5 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -97,6 +97,9 @@
 # and host information.
 include $(BUILD_SYSTEM)/config.mk
 
+# Default soong to on
+USE_SOONG ?= true
+
 ifndef KATI
 ifdef USE_NINJA
 $(warning USE_NINJA is ignored. Ninja is always used.)