Export SOONG_DEFINED_SYSTEM_IMAGE_PATH to make

This makes the make packaging system resilient to filename changes.

Test: lunch aosp_cf_x86_64_phone-userdebug && m systemimage
Change-Id: I166d78cd49f72a875b9648ca9285f7ebc2307a76
diff --git a/filesystem/filesystem.go b/filesystem/filesystem.go
index 9f2b239..ecc1524 100644
--- a/filesystem/filesystem.go
+++ b/filesystem/filesystem.go
@@ -1373,3 +1373,9 @@
 		return true
 	})
 }
+
+func (f *filesystem) MakeVars(ctx android.MakeVarsModuleContext) {
+	if f.Name() == ctx.Config().SoongDefinedSystemImage() {
+		ctx.StrictRaw("SOONG_DEFINED_SYSTEM_IMAGE_PATH", f.output.String())
+	}
+}