With the split CMakeLists.txt infrastructure avrdude.conf
will be created in the build/src and not build folder. Hence,
fix its location in the install command.
The main CMakeLists.txt file in the project's root directory takes
care of the main project settings like project name and version,
handling the options, finding dependencies, etc.
The src/CMakeLists.txt handles options that are necessary to build
libavrdude library and avrdude binary.
Since commit ed46f09c1ccd1351e003a200ba50e3e4778ac478 (Implement
tc[io]flush methods & deprecate broken purge_buffers methods.)
ftdi_usb_purge_buffers() routine is deprecated. Use HAVE_FTDI_TCIOFLUSH
macro to invoke the newly introduced ftdi_tcioflush() routine.