Add brillo cflags

Add __BRILLO__ to global cflags when building for a brillo product.

Bug: 26165350
Change-Id: I0100a8821b763075d1873d0d48fd5bd217afb580
diff --git a/common/variable.go b/common/variable.go
index 2c0e0a6..4f159c5 100644
--- a/common/variable.go
+++ b/common/variable.go
@@ -82,6 +82,7 @@
 	CrossHostSecondaryArch *string `json:",omitempty"`
 
 	Unbundled_build *bool `json:",omitempty"`
+	Brillo          *bool `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {