cmd: avb2.0: avb command for performing verification

cmd: avb2.0: avb command for performing verification

Enable a "avb" command to execute Android Verified
Boot 2.0 operations. It includes such subcommands:
  avb init - initialize avb2 subsystem
  avb read_rb - read rollback index
  avb write_rb - write rollback index
  avb is_unlocked - check device lock state
  avb get_uuid - read and print uuid of a partition
  avb read_part - read data from partition
  avb read_part_hex - read data from partition and output to stdout
  avb write_part - write data to partition
  avb verify - run full verification chain

Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>