The URL to fetch the configuration for a talos node is given by the
talos.config kernel parameter. We add support for 4 variables ${uuid},
${serial}, ${mac} and ${hostname} which substitute the device UUID,
DMI-sourced serial number, MAC address of the first network interface to
be up and the hostname respectively.
Fixes#3272
Signed-off-by: Philipp Sauter <philipp.sauter@siderolabs.com>
