fix building TWRP for devices without cache

Some devices do not declare BOARD_CACHEIMAGE_PARTITION_SIZE.
This case is detected and required modules are added to LOCAL_REQUIRED_MODULES.
However, := is used instead of +=  and causes the loss of all previously added modules.
The end result is that twrp is not actually getting built since its module was lost.
Fix that by using += when adding modules required for cache-less devices.

Change-Id: Ib0a4db4955040ced77be7ed04d636905beadf093

Android.mk

@@ -500,7 +500,7 @@ ifeq ($(shell test $(PLATFORM_SDK_VERSION) -ge 25; echo $$?),0)
 endif
 
 ifeq ($(BOARD_CACHEIMAGE_PARTITION_SIZE),)
-LOCAL_REQUIRED_MODULES := recovery-persist recovery-refresh
+LOCAL_REQUIRED_MODULES += recovery-persist recovery-refresh
 endif
 
 include $(BUILD_EXECUTABLE)