FreeModbus library rework.

This is a rework of freemodbus library for embedded systems (http://freemodbus.org/) originally written by Christian Walter wolti@sil.at.

Goals of this rework:

• Replace hungarian notation in code to respond Linux kernel coding guidelines
• Update AVR port (since it uses poisoned code)
• Create STM32 port
• Make library useable with Antares build environment (https://github.com/nekromant/antares)

What is currently done and need testing:

• Updated AVR port. Now it uses ISR instead of SIGNAL interrupt syntax.
• Added Antares integration. To work with it, just run bootstrap (if you have Antares globally installed)