AVB: support chain partition signing

Current build system will include AVB metadata from each partition and
store them into /vbmeta partiton when BOARD_AVB_ENABLE is set, which makes
each partition tightly-coupled.

Add the support for 'chain partition':
  - The vbmeta of each partition is stored on the same partition itself.
  - The public key used to verify each partition is stored in /vbmeta.

For example, the following build variables are required to enable chain
partition for system partition:
  - BOARD_AVB_SYSTEM_KEY_PATH := path/to/system_private_key
  - BOARD_AVB_SYSTEM_ALGORITHM := SHA512_RSA8192
  - BOARD_AVB_SYSTEM_ROLLBACK_INDEX := 1
  - BOARD_AVB_SYSTEM_ROLLBACK_INDEX_LOCATION := 2

The corresponding settings will be added into META/misc_info.txt for
build_image.py and/or add_img_to_target_files.py:
  - avb_system_key_path=path/to/system_private_key
  - avb_system_algorithm=SHA512_RSA8192
  - avb_system_add_hashtree_footer_args=--rollback_index 1
  - avb_system_rollback_index_location=2

To enable chain partition for other partitions, just replace SYSTEM with
BOOT, VENDOR and/or DTBO in the build variables.

Also switch from  `avbtool make_vbmeta_image --setup_rootfs_from_kernel system.img ...`
to `avbtool add_hashtree_footer --image system.img --setup_as_rootfs_from_kernel...`
when BOARD_BUILD_SYSTEM_ROOT_IMAGE is true. This works for both chained
and no-chained:
  - chained: `avbtool add_hashtree_footer --setup_as_rootfs_from_kernel` will
    add dm-verity kernel cmdline descriptor to system.img
  - no-chained: `avbtool make_vbmeta_image --include_descriptors_from_image
    system.img` will include the kernel cmdline descriptor from system.img into
    vbmeta.img

Bug: 38399657
Test: `make` pass, flash images from $OUT and boot device without chain partitions
Test: `make` pass, flash images from $OUT and boot device with chain partitions
Test: `make dist` pass, flash images from TF.zip and boot device without chain partitions
Test: `make dist` pass, flash images from TF.zip and boot device with chain partitions
Test: follow the same steps in
      https://android-review.googlesource.com/#/c/407572/

Change-Id: I344f79290743d7d47b5e7441b3a21df812a69099