Build cache.img on demand

Bug: 5153694
To build cache.img, set BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE (required,
ext4 only for now), BOARD_CACHEIMAGE_PARTITION_SIZE (optional) in
BoardConfig.mk.

Change-Id: I1d8b91646aa1dba88285e008ad3335768bcbddd2
diff --git a/core/product.mk b/core/product.mk
index 26f365d..89c9e7b 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -227,6 +227,8 @@
 	BOARD_RECOVERYIMAGE_PARTITION_SIZE \
 	BOARD_SYSTEMIMAGE_PARTITION_SIZE \
 	BOARD_USERDATAIMAGE_PARTITION_SIZE \
+	BOARD_CACHEIMAGE_FILE_SYSTEM_TYPE \
+	BOARD_CACHEIMAGE_PARTITION_SIZE \
 	BOARD_FLASH_BLOCK_SIZE \
 	BOARD_SYSTEMIMAGE_PARTITION_SIZE \
 	BOARD_VENDOR_QCOM_GPS_LOC_API_HARDWARE \