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 `NETSTACK_CONF_BRIDGE_MODE` to 1. Disabled by
default.
Add mesh_ready() to mesh
mesh_ready checkes whether a packet is queued. This allows to avoid
packet loss at application level.
Merge pull request #75 from cmorty/mesh_ready

Add mesh_ready() to mesh
Merge pull request #135 from darconeous/pull-requests/core-net-bridge-mode

core/net: Added support for "bridge mode".
Merge remote-tracking branch 'upstream/master'