Set a dummy CM_PLATFORM_SDK_VERSION if it doesn't exist

* Avoids printing 2 build warnings on every single rebuild
  of anything when building TWRP inline

Change-Id: I5dd3623707c03af0dfe7366f1a77e83762e720b2

Android.mk

@@ -34,6 +34,10 @@ ifneq (,$(filter $(PLATFORM_SDK_VERSION), 21 22))
     BOARD_SEPOLICY_UNION += twrp.te
 endif
 
+ifeq ($(CM_PLATFORM_SDK_VERSION),)
+    CM_PLATFORM_SDK_VERSION := 0
+endif
+
 include $(CLEAR_VARS)
 
 TWRES_PATH := /twres/