native periph rtc

* implement `periph/rtc` instead of the [deprecated](https://github.com/RIOT-OS/RIOT/pull/1919) `rtc` in native
* add error checking / warnings
* clean up the code a bit

Edit: split all non-native parts into separate PRs.
https://github.com/RIOT-OS/RIOT/pull/1922/files#discussion_r20663475