Draft: Add option to change ethernet MAC address before IP setup

Add option for ethernet MAC change. This allows functionality without VLAN when boot-up MAC address is common over hosts or is not set permanent.

For per-interface option, use b2c.iface mac=: this tries to set MAC address before IP setup (auto or manual).

For global option, use 'mac=' command line parameter: this tries to set MAC address before looking for DHCP information through link-up interfaces.

Current naming for the global option is unfortunate, but it's something I need to live with. Martin approved leaving it as an alias after better naming option has been decided.