AVR Serial Library
Hardware UART library for many AVR MCUs
Project started on October 15, 2012.Last updated on September 15, 2017.
Recent activity on GitHub: 
The avrSerial Library enables interrupt-driven UART communication on all available UART modules. Each module has it's own receive and transmit buffer. XON/XOFF Flow Control for the receiving end can be enabled. At the moment, the following AtMega types are supported:
AtMega8
AtMega16
AtMega32
AtMega8515
AtMega8535
AtMega323
AtMega168
AtMega328
AtMega48
AtMega88
AtMega168p
AtMega328p
AtMega48p
AtMega88p
AtMega2561
AtMega1281
AtMega1284p
AtMega2560
AtMega1280
AtMega640
AtTiny2313
AtTiny2313a
AtTiny4313
AtXMega128a1
To add another processor, just get the register names from the data sheet and put them in the header file.