Makefile: Define TARGET_USE_DTBO flag

Add TARGET_DTBO_NOT_SUPPORTED flag. This is
needed by avb to determine dtbo image validation.

Change-Id: I048e5eb7f547eeb3ebc1c152432f067adbf1d04c
diff --git a/makefile b/makefile
index b2d40a6..f6da064 100644
--- a/makefile
+++ b/makefile
@@ -127,6 +127,10 @@
   DEFINES += _SIGNED_KERNEL=1
 endif
 
+ifeq ($(TARGET_DTBO_NOT_SUPPORTED),1)
+  DEFINES += TARGET_DTBO_NOT_SUPPORTED=1
+endif
+
 ifeq ($(OSVERSION_IN_BOOTIMAGE),1)
  DEFINES += OSVERSION_IN_BOOTIMAGE=1
 endif