This adds a custom build script for 'hana' ('oak') that doesn't rely on
Docker, as the cros_sdk for oak doesn't support the --nouse-image flag.

The script (cros_build/build_hana.sh) takes care of downloading the
sources and creating a sdk chroot appropriately. Once inside the chroot,
to build the firmware run:

  ./hana.sh setup
  ./hana.sh checkout
  ./hana.sh build

and the resulting firmware binary will be in
cros_build/build_hana/hana.bin.

All the downloaded and generated files will be contained in the
cros_build/build_hana directory.

This also includes the custom firmware binary for
hana (firmware/hana.bin), although it wasn't tested.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>