Support to configure and build multiple custom images.

Build additional images requested by the product makefile.
This script gives the ability to build multiple additional images and
you can configure what modules/files to include in each image.
1. Define PRODUCT_CUSTOM_IMAGE_MAKEFILES in your product makefile.
   PRODUCT_CUSTOM_IMAGE_MAKEFILES is a list of makefiles.
   Each makefile configures an image.
   For image configuration makefile foo/bar/xyz.mk, the built image
   file name
   will be xyz.img. So make sure they won't conflict.
2. In each image's configuration makefile, you can define variables:
  - CUSTOM_IMAGE_MOUNT_POINT, the mount point, such as "oem", "odm"
    etc.
  - CUSTOM_IMAGE_PARTITION_SIZE
  - CUSTOM_IMAGE_FILE_SYSTEM_TYPE
  - CUSTOM_IMAGE_DICT_FILE, a text file defining a dictionary
    accepted by BuildImage() in tools/releasetools/build_image.py.
  - CUSTOM_IMAGE_MODULES, a list of module names you want to include
    in the image; Not only the module itself will be installed to proper
    path in the image, you can also piggyback additional files/directories
    with the module's LOCAL_PICKUP_FILES.
  - CUSTOM_IMAGE_COPY_FILES, a list of "<src>:<dest>" to be copied to
    the image. <dest> is relativ to the root of the image.

To build all those images, run "make custom_images".

Bug: 19609718
Change-Id: Ic73587e08503a251be27797c7b00329716051927
(cherry picked from commit 5fcf1094)