wireguard-tools: allow generating private_key

wireguard-tools: allow generating private_key

When the uci configuration is created automatically during a very early
stage, where no entropy daemon is set up, generating the key directly is
not an option. Therefore we allow to set the private_key to "generate"
and generate the private key directly before the interface is taken up.

Signed-off-by: Leonardo Mörlein <me@irrelefant.net>
Tested-by: Jan-Niklas Burfeind <git@aiyionpri.me>