New realtime-modbus driver for Mesa cards

I have just pushed a feature branch (andypugh/mesa_modbus) to the LinuxCNC github. 

It uses the Mesa FPGA UARTs (specifically pktUART) to offer real-time modbus connection directly between a Mesa card and a Modbus device.

Docs are here: github.com/LinuxCNC/linuxcnc/blob/andypu...ers/mesa_modbus.adoc

Please let me know of anything that you feel you need to know that the docs omit. 

At the moment you will find mesa_modbus.c and the modcompile script in linuxcnc-dev/src/hal/drivers/mesa_hostmot2/modbus along with some sample configuration files. 

Pin assignments and names are compiled-in rather than being determined at load time.