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/main.mk b/core/main.mk
index e35f228..97d3d7e 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -743,6 +743,9 @@
 .PHONY: userdatatarball
 userdatatarball: $(INSTALLED_USERDATATARBALL_TARGET)
 
+.PHONY: cacheimage
+cacheimage: $(INSTALLED_CACHEIMAGE_TARGET)
+
 .PHONY: bootimage
 bootimage: $(INSTALLED_BOOTIMAGE_TARGET)
 
@@ -757,6 +760,7 @@
 	$(INSTALLED_BOOTIMAGE_TARGET) \
 	$(INSTALLED_RECOVERYIMAGE_TARGET) \
 	$(INSTALLED_USERDATAIMAGE_TARGET) \
+	$(INSTALLED_CACHEIMAGE_TARGET) \
 	$(INSTALLED_FILES_FILE)
 
 # dist_files only for putting your library into the dist directory with a full build.