This initial version of `caterpillar` can detect and install compatible
RAUC update bundles in top-level directories on attached, compatible
block devices.
Caterpillar automatically selects the compatible update bundle with the
highest version and attempts to upgrade to it.
Currently only semver versions are supported!

By placing a single update bundle in a directory called `override` (name
is configurable), caterpillar ignores other detected update bundles and
the version check. This allows downgrade scenarios.

For discovery and installation, caterpillar communicates with udisks2
and rauc over dbus. The other services take care of mounting block
devices and retrieving information on discovered RAUC bundles.

By default caterpillar triggers a reboot after successful update (by
connecting with logind over dbus). This behavior is configurable.

An example systemd service for starting caterpillar during boot is
provided.