mender-artifact can modify any file

Today we can modify these configuration items using mender-artifact modify:

   --key value, -k value               Full path to the private key that will be used to sign the artifact after modifying.
   --server-uri value, -u value        Mender server URI; the default URI will be replaced with given one.
   --server-cert value, -c value       Full path to the certificate file that will be used for validating Mender server by the client.
   --verification-key value, -v value  Full path to the public verification key that is used by the client  to verify the artifact.
   --name value, -n value              New name of the artifact.
   --tenant-token value, -t value      Full path to the tenant token that will be injected into modified file.

For evaluation and testing purposes it is very beneficial to enable mender-artifact modify to modify any file within the disk image (.sdimg) or Mender Artifact (.mender). In particular, the tutorial for physical devices require to update the network configuration:

• Append entries to /etc/hosts
• Replace file /etc/systemd/network/eth.network

It would drastically simplify this getting started guide if these two items were supported directly in mender-artifact modify.

A simple user interface to this would be to allow the user to run any command that reads or writes to the root file system. For example:

mender-artifact modify mender-raspberrypi3_master.sdimg --command "echo $IP_OF_MENDER_SERVER_FROM_DEVICE docker.mender.io s3.docker.mender.io |  tee -a $MENDER_ROOTFS/etc/hosts"

Another option is to run a command that produces some output and explicitly specify a file that this output should go into; however this might be a bit more complicated (may need to support append and set to file, for example).

Acceptance criteria

• Any file in the rootfs can be modified
• The referenced documentation is simplified