Remove wiring pi code that's not needed in our FPP repository.
Fix a crash if the settings file open fails.
Update Makefile and stub pixelnet code

* Now build will auto-detect if you're building on a PI, or a generic
  i686 linux machine.  For Pi it will build pixelnetDMX code in which
  requires wiringPi.  For generic i686 builds stubs will be used instead
  and this functionality will simply log when called.
Remove warnings from the build in E131.c and e131bridge.c