Added stubs for CAN_AT90CAN{.cpp,.h}
Based on work by Fabian Greif for the initial AT90CAN library from https://github.com/dergraaf/avr-can-lib
Added stubs for CAN_SJA1000{.cpp,.h}
Based on work by Fabian Greif for the initial SJA1000 library from https://github.com/dergraaf/avr-can-lib
Added stubs for CAN_K2X{.cpp,.h}
Based on work by teachop & branduino for the initial CAN_K2X library from https://github.com/teachop/FlexCAN_Library