udev: link-config - add mac address policy

This introduces a new key MACAddressPolicy.

The possible policies are 'persistent' and 'random'.

'persistent' will do nothing if the current address is the hardware address,
but if the hardware does not have an address (or another address is set for
whatever reason), we will generate an address which will be random, but
persistent between boots (based on machineid and persistent netif name).

'random' will do nothing if the kernel already set a random address, otherwise
it will generate a random one and use that instead.

This patch sets MACAddressPolicy=persistent in the default .link file.