Set default value for TW_MAX_BRIGHTNESS if needed

TW_MAX_BRIGHTNESS flag is required if TW_NO_SCREEN_BLANK is set.
Set TW_MAX_BRIGHTNESS default value only if TW_NO_SCREEN_BLANK is set and TW_MAX_BRIGHTNESS is not set in the device tree.

Display build warning regarding default value being set.

Change-Id: I6a4803ef873b05814f1b4487217d0d1d05cacd6c
diff --git a/Android.mk b/Android.mk
index 1d45095..ae4b721 100644
--- a/Android.mk
+++ b/Android.mk
@@ -384,7 +384,8 @@
 	LOCAL_CFLAGS += -DTW_MAX_BRIGHTNESS=$(TW_MAX_BRIGHTNESS)
 else
     ifneq ($(TW_NO_SCREEN_BLANK),)
-        $(call pretty-error,TW_MAX_BRIGHTNESS must be defined if using TW_NO_SCREEN_BLANK. Please update your device tree to include this flag with the correct value from your device.)
+        TW_MAX_BRIGHTNESS ?= 1023
+        $(call pretty-warning,TW_MAX_BRIGHTNESS must be defined if using TW_NO_SCREEN_BLANK. Setting to default value: $(TW_MAX_BRIGHTNESS). If you would like to use a different value then please update your device tree with the correct value from your device.)
     endif
 endif
 ifneq ($(TW_DEFAULT_BRIGHTNESS),)