backup: support exclusion of user-defined directories from backups

Example: TW_BACKUP_EXCLUSIONS := /data/fonts,/data/blah,/data/geewhiz

Change-Id: I99e690eb0879281b1149e60ff430b6b3e7dd93c2
(cherry picked from commit cc683e7a3deefb96ece49b07bf88648087a7ccaf)
diff --git a/Android.mk b/Android.mk
index 84dc6be..0d2708d 100755
--- a/Android.mk
+++ b/Android.mk
@@ -408,6 +408,9 @@
 ifneq ($(TARGET_OTA_ASSERT_DEVICE),)
     LOCAL_CFLAGS += -DTARGET_OTA_ASSERT_DEVICE='"$(TARGET_OTA_ASSERT_DEVICE)"'
 endif
+ifneq ($(TW_BACKUP_EXCLUSIONS),)
+	LOCAL_CFLAGS += -DTW_BACKUP_EXCLUSIONS='"$(TW_BACKUP_EXCLUSIONS)"'
+endif
 
 LOCAL_C_INCLUDES += system/vold \