native periph rtc

* implement `periph/rtc` instead of the [deprecated](https://github.com/RIOT-OS/RIOT/pull/1919) `rtc` in native
* add `periph_rtc` to `auto_init`
* add `periph_rtc` to `FEATURES_OPTIONAL` in `examples/default`

**missing:**
* shell command
 * I could not figure out how to get `sys/shell/commands/Makefile` to work with something like `$(filter periph_rtc,$(FEATURES_PROVIDED))`, it just didn't work
* `tests/periph_rtc`
 * will address when shell command is in place
* part of the API is not implemented
 * will address when shell command is in place