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.
Reduced code size.

Combined recent changes from darconeous...
- Refactor to decrease minimum code footprint.
- Added `RESOLV_CONF_SUPPORTS_RECORD_EXPIRATION`.
...with a few additional changes to reduce code size.
Merge pull request #166 from oliverschmidt/master

Reduced code size.
Added configurations of the new resolver.
Merge pull request #167 from oliverschmidt/master

Added configurations of the new resolver.
core/net/resolv: Fix for SDCC.
core/net/resolv: Fix for resolving CNAME records.
core/net/resolv: Misc. updates and cleanups.
Merge pull request #168 from darconeous/pull-requests/core-net-resolv-misc

Miscellaneous small DNS resolver updates and fixes
Restored build in cmd.exe environmet by re-applying change https://github.com/contiki-os/contiki/commit/f3cf57e41b277beebd353a4d76aba2359868b361 which was reverted recently.
Merge pull request #169 from oliverschmidt/master

Restored build in cmd.exe environment.
Fix a syntax error in /cpu/cc253x/dev/uart0.c
[ci] Travis needs 32-bit compatibility libraries
Merge pull request #171 from errordeveloper/fix_cs_toolchain_install_script_on_travis

[ci] Travis needs 32-bit compatibility libraries
Merge pull request #170 from RongHQ/master

Fix a syntax error in cpu/cc253x/dev/uart0.c
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".