Add support for Brillo Verified Boot.

The following variables are introduced

 BOARD_BVB_ENABLE: can be set to true to build boot.img and system.img
 files compatible with Brillo Verfied Boot.

 BOARD_BVB_ROLLBACK_INDEX: can be set to an integer to use for the
 rollback index.

 BOARD_BVB_KEY_PATH, BOARD_BVB_ALGORITHM: If set, the former must be a
 path to the private key used to sign the boot image and the latter must
 be the algorithm to use. If unset, a test-key stored in the tree will
 be used.

 BOARD_BVB_MAKE_BOOT_IMAGE_ARGS: Extra options to pass to 'bvbtool
 make_boot_image'.

 BOARD_BVB_SIGN_BOOT_IMAGE_ARGS: Extra options to pass to 'bvbtool
 sign_boot_image'.

 BOARD_BVB_ADD_IMAGE_HASHES_ARGS: Extra options to pass to 'bvbtool
 add_image_hashes'.

 BOARD_CUSTOM_BVBTOOL: Can be set to specify what bvbtool program to
 use.

The existing BOARD_KERNEL_CMDLINE variable is also used, as are existing
kernel and initrd-related variables. Therefore, simply adding

 BOARD_BVB_ENABLE := true

to an existing Makefile should do the trick.

Bug: 26185038
TEST=Added 'BOARD_BVB_ENABLE := true' to hardware/bsp/intel/soc/edison/soc.mk
  and built an image and then ran bvbtool's info_boot_image and
  info_image_hashes commands on the resulting boot.img and system.img
  files and verified that the information was correct. Also ran 'm dist'
  and verified that the boot.img and system.img files in the resulting
  target_files.zip file had similar information.

Change-Id: I08045ed8b0cbddc7c3acdd3a6f2c4bb75cb44bbc