package/mender-artifact: new package

package/mender-artifact: new package

mender-artifact is a host tool to generate update images
in the Mender artifact file format.
This package uses the binary archive from github because it bundles the
external dependencies.

Example usage:

    $ mender-artifact write rootfs-image \
         --update rootfs.ext4 \
         --output-path rootfs.mender \
         --artifact-name "release-v1.0.0" \
         --device-type "beaglebone"

Above will generate a Mender artifact called "rootfs.mender"
containing the "rootfs.ext4" image along with meta-data.

One can read-out the meta-data with the following command:

    $ mender-artifact read rootfs.mender
      Mender artifact:
      Name: release-v1.0.0
      Format: mender
      Version: 2
      Signature: no signature
      Compatible devices: '[beaglebone]'

    Updates:
      0000:
        Type:   rootfs-image
        Files:
          name:     rootfs.ext4
          size:     52428800
          modified: 2018-08-27 09:10:55 +0200 CEST
          checksum: e70b113fb0964a810a3043586eb4fc1c48e684ba78b02ba65fead4aa3e540d87

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Tested-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>