[netdevice] Allow driver to preinitialise the link-layer address

[netdevice] Allow driver to preinitialise the link-layer address

Drivers are currently expected to initialise only the hardware
address, with the link-layer protocol code taking care of converting
this into a valid link-layer address.  Some drivers (e.g. undinet) can
legitimately determine both the hardware and link-layer addresses,
which may differ.

Allow for this situation by checking to see if the link-layer address
is empty before initialising it from the hardware address.

Signed-off-by: Michael Brown <mcb30@ipxe.org>