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: I3778e3d091979261cb476628da1365f931e11f49