core/net: Added support for "bridge mode".

"Bridge mode" allows devices to more easily send 802.15.4 packets as if they were a different device. It also turns off any packet filtering that may be implemented at layer 2. It works by allowing `PACKETBUF_ADDR_SENDER` to be set earlier in the stack.

This is useful for implementing 6LoWPAN-ethernet bridges.

Enabled via setting `CONTIKI_CONF_MAC_BRIDGE_MODE` to 1. Disabled by default.