brillo: Add tool for creating A/B metadata misc images.

This tool creates a misc image containing the A/B metadata to be used by
an A/B aware bootloader. It is to be placed in the "misc" partition of a
final disk image.

BUG=29072958
TEST=Unit tests covering input of various metadata attribute values.

Change-Id: Ic9d8b5813c39556fbc3dedc7325bd90b92bb1ff3