[build][fix] Should only use `XI_CFLAGS` internally -

When `XI_EXTRA_CFLAGS` is used intenrally any external settings
will override it, e.g. running `make XI_EXTRA_CFLAGS=-I../../libfoo`
breaks all internal assignments to `XI_EXTRA_CFLAGS`.
[io][wiznet] Added a prototype implementation

To build it run:
make -C src/ XI_IO_LAYER=wiznet \
  CC=avr-gcc AR=avr-ar \
  XI_OPTIMISE=NO_ERROR_STRINGS \
  XI_DEBUG_OUTPUT=0 \
  XI_DEBUG_ASSERT=0 \
  XI_BUILD_TYPE=release \
  XI_EXTRA_CFLAGS="-I/home/ilya/libxively-avr-wiznet/gateway_and_device/apps/Gateway/tcpip/ -mmcu=atmega256rfr2"