Implement support for automatic bootstrap artifact in client

Acceptance criteria:

• When starting, Mender client looks for a special artifact, /var/lib/mender/bootstrap.mender, and installs it if present
• Artifact is deleted after being installed
• If database already exists, bootstrap artifact must be deleted without being installed
  • This is important to make sure that if you upgrade from a client without bootstrap artifact support, to one that has it, it doesn't install the artifact at that time
• Signature support must be disabled while installing this particular artifact
  • This is to save the user from signing an artifact that he didn't ask for. It is an implementation detail from our side
  • Make sure it only applies to the exact location /var/lib/mender/bootstrap.mender (possibly with symlinks), and nowhere else
• Drop support for reading /etc/mender/artifact_info.
• Remove installation of artifact_info from under install-conf in the Makefile. Keep the install-conf target though. It's a no-op now, but for packaging purposes it's better to keep it around.
• If bootstrap artifact doesn't exist, and the database doesn't contain artifact_name, automatically populate the database with artifact_name = unknown.

Feature branch: automatic_bootstrap_artifact