Add Makefile to use when building with GCC
Closer to gcc build

- Add linker-script
- Add UIP sources to Makefile
- Add extern "C" around uip includes
- Add dummy functions needed to link C++ code
- Remove static declaration from ENC28J60 instance as it causes GCC to
  use _cxa_guard_acquire() and _cxa_guard_release(), which we don't have.