Merge pull request #1 from pqqp/tmp

Fix Keil warning: #61-D
Uart: add TXC flag check function and minor fix

Add function TxDone() to check TXC flag, it's useful for RS485 direction
control in polling mode.

Add ATmega64 to Error enum.
Minor fixes of Atmega64 timer and AVR usart
Modbus RTU implementation
Add Avr Modbus slave example
Merge pull request #4 from pqqp/master

Add Modbus RTU master/slave implementation + minor fixes