rs485-extension: RS485 master implementation for brickd running on RED-brick.

In primary testing it works but needs more testing and cleaning up of the code.
Merge branch 'master' of github.com:Tinkerforge/brickd

Conflicts:
	src/brickd/rs485_extension.c
rs485-extension: RS485 extension for brickd running in red-brick.

Primary tests worked but needs some more testing and cleaning up of the code.