Support to build factory ramdisk image.

Bug: 5769921

With this change, to build factory_ramdisk.img, set
PRODUCT_FACTORY_RAMDISK_MODULES in your product config.
PRODUCT_FACTORY_RAMDISK_MODULES consists of
"<module_name>:<install_path>" pairs.
<install_path> is relative to the root of the factory ramdisk output.
For example:
PRODUCT_FACTORY_RAMDISK_MODULES := \
    toolbox:bin/toolbox adbd:sbin/adbd adb:bin/adb

On the other hand you can use PRODUCT_COPY_FILES to copy prebuilt files
to the factory ramdisk.
Or you can define modules that are specific for the factory ramdisk
(with LOCAL_MODULE_PATH pointing to TARGET_FACTORY_RAMDISK_OUT) and add
the module names to PRODUCT_PACKAGES.

Change-Id: I80ff72606415cd74fe6f3bc93020a05d84e45b70
diff --git a/core/main.mk b/core/main.mk
index 97d3d7e..48639b5 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -727,6 +727,9 @@
 .PHONY: ramdisk
 ramdisk: $(INSTALLED_RAMDISK_TARGET)
 
+.PHONY: factory_ramdisk
+factory_ramdisk: $(INSTALLED_FACTORY_RAMDISK_TARGET)
+
 .PHONY: systemtarball
 systemtarball: $(INSTALLED_SYSTEMTARBALL_TARGET)
 
@@ -807,6 +810,7 @@
     $(BUILT_TARGET_FILES_PACKAGE) \
     $(INSTALLED_ANDROID_INFO_TXT_TARGET) \
     $(INSTALLED_RAMDISK_TARGET) \
+    $(INSTALLED_FACTORY_RAMDISK_TARGET) \
    )
 
 # Building a full system-- the default is to build droidcore