[PlatformWithOS] #11 Provide a way to change I/O arrangement

Include an (RPi) example file: epd_io_free_uart.h
(that leaves the UART pins free)

Command Sequence: (note: alternative I/O file has no path prefix
                         it must be in the appropriate directory)

% make rpi-clean
% make EPD_IO=epd_io_free_uart.h rpi
% sudo make EPD_IO=epd_io_free_uart.h rpi-install

Signed-off-by: Christopher Hall <christopherhall.hsw@gmail.com>