Support system/vendor for devices without a vendor partition

Test: lunch aosp_arm-eng; m -j vulkan.default
Change-Id: I91eabf98f53c455ec4ade95215f11a6f89631747
diff --git a/android/variable.go b/android/variable.go
index 3436a75..1ee6c9b 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -113,6 +113,8 @@
 	UseGoma                    *bool `json:",omitempty"`
 	Debuggable                 *bool `json:",omitempty"`
 
+	VendorPath *string `json:",omitempty"`
+
 	ClangTidy  *bool   `json:",omitempty"`
 	TidyChecks *string `json:",omitempty"`