Add -Wall -Werror to CFLAGS

Previously, we missed out on compiler warnings that were actual bugs.

Change-Id: Icc1e09c29536433de2e36afa33709c324455a44a
diff --git a/libupgrade/Makefile b/libupgrade/Makefile
index 8c28db9..df9c334 100644
--- a/libupgrade/Makefile
+++ b/libupgrade/Makefile
@@ -9,7 +9,7 @@
 RANLIB:=$(CROSS_PREFIX)ranlib
 STRIP:=$(CROSS_PREFIX)strip
 
-CFLAGS = -g -Os
+CFLAGS = -g -Os -Wall -Werror
 
 SRCS = hmx_upgrade_flash.c hmx_upgrade_nvram.c
 INCS = hmx_upgrade_nvram.h hnvram_dlist.h