Use ${WERROR} consistently.

Change-Id: I37b414b030cc69fab2b4feec70b55297dcd5fdb7
Reviewed-on: https://chromium-review.googlesource.com/313198
Commit-Ready: Doug Evans <dje@google.com>
Tested-by: Doug Evans <dje@google.com>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
diff --git a/Makefile b/Makefile
index 0a2c23c..d066916 100644
--- a/Makefile
+++ b/Makefile
@@ -154,7 +154,7 @@
 else
 # FIRMWARE_ARCH not defined; assuming local compile.
 CC ?= gcc
-CFLAGS += -DCHROMEOS_ENVIRONMENT -Wall -Werror ${DEBUG_FLAGS}
+CFLAGS += -DCHROMEOS_ENVIRONMENT -Wall ${WERROR} ${DEBUG_FLAGS}
 endif
 
 ifneq (${DEBUG},)