soong: print ROM_BUILDTYPE in build banner

Change-Id: I17d9df08f79818aecb40b165cd063bbc5f01e559
diff --git a/ui/build/dumpvars.go b/ui/build/dumpvars.go
index eba86a0..3d43786 100644
--- a/ui/build/dumpvars.go
+++ b/ui/build/dumpvars.go
@@ -164,6 +164,7 @@
 	"BUILD_ID",
 	"OUT_DIR",
 	"SOONG_SDK_SNAPSHOT_TARGET_BUILD_RELEASE",
+	"ROM_BUILDTYPE",
 }
 
 func Banner(make_vars map[string]string) string {