WIP: Native GPIO

- dummy and Linux sysfs GPIO drivers for native
- minor PIR driver test fix

TODO:
- [ ] add signal mode for sysfs driver
- [x] implement writing for sysfs driver
- [x] test sysfs driver on raspberry PI
- [ ] add control method for virtual interface
- [x] address sysfs GPIO availability issue
- [x] guard native system calls
- [x] rename "dummy" to "virtual"?

~~- [ ] enable pullup/pulldown on sysfs (if possible)~~ not posssible
~~- [ ] enable mapping of host GPIO numbers to native GPIO numbers~~

External GPIO/sysfs documentation:
http://pi.gadgetoid.com/pinout
http://wiki.openwrt.org/doc/hardware/port.gpio

~~Based on https://github.com/RIOT-OS/RIOT/pull/1838~~
hm, above you define, that there can be up to 32 GPIOs, but here you stop at 15...