examples/webserver: Add MDNS resolver (Bonjour) support.

This allows you to access the web server by simply visiting
<http://contiki.local./>, instead of needing to enter in the
IP address manually.
cooja: Allow CFLAGS to filter thru
cpu/avr/dev/usb: Various minor updates.
Misc AVR updates.
cpu/cc253x: Miscellaneous compiler fixes.
cpu/mc1322x: Minor changes not sure if they are worth keeping.
Add to native-target-osx branch.