modbus: add MODBUS RTU subsystem

Add MODBUS RTU (over serial line) subsystem.
MODBUS RTU implementation supports booth server and
client roles. Some components of the implementation are based
on the uC/Modbus stack, which was published under Apache license,
(https://github.com/SiliconLabs/uC-Modbus


 fdd1218a28e313c1212fed5ed42e5c65d3056a2c).

Resolves: #2854

Signed-off-by: Johann Fischer <j.fischer@phytec.de>