make: Add VERIFIED_BOOT_2 flag to make files

Add new flag to make files to Enable AVB2.0 compilation if
global flag BOARD_AVB_ENABLE is enabled from target specific
folder.

Change-Id: I4f907c5c5c7b6709bab70b599b1180039277a26e
diff --git a/makefile b/makefile
index 156ebeb..df6e89b 100644
--- a/makefile
+++ b/makefile
@@ -122,6 +122,10 @@
     DEFINES += DEFAULT_UNLOCK=1
   endif
 endif
+ifeq ($(VERIFIED_BOOT_2),1)
+  DEFINES += VERIFIED_BOOT_2=1
+  DEFINES += _SIGNED_KERNEL=1
+endif
 
 ifeq ($(OSVERSION_IN_BOOTIMAGE),1)
  DEFINES += OSVERSION_IN_BOOTIMAGE=1