project: enable battery voltage checking

Enable battery voltage checking before flashing or erasing
image.

Change-Id: Ie95f3b88d20b19cea711f9cef980590ceed0ff89
diff --git a/project/msm8994.mk b/project/msm8994.mk
index 8016203..db7d8f0 100644
--- a/project/msm8994.mk
+++ b/project/msm8994.mk
@@ -81,3 +81,6 @@
 
 # Turn on Werror
 CFLAGS += -Werror
+
+#enable battery voltage check
+DEFINES += CHECK_BAT_VOLTAGE=1