Blacklist DISPLAY / GREP_OPTIONS

We keep having makefiles reading $DISPLAY, and GREP_OPTIONS with
--color=always has caused problems with shell commands in the past.

Bug: 37333696
Test: GREP_OPTIONS="--color=always" m -j
Test: DISPLAY=test m -j (modify an Android.mk to check DISPLAY)
Change-Id: Iafab37d61ecde5c6fdb35a05f18709abc17e8554
diff --git a/ui/build/config.go b/ui/build/config.go
index 9a8a3fb..c01bf4c 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -83,6 +83,10 @@
 
 		// We handle this above
 		"OUT_DIR_COMMON_BASE",
+
+		// Variables that have caused problems in the past
+		"DISPLAY",
+		"GREP_OPTIONS",
 	)
 
 	// Tell python not to spam the source tree with .pyc files.