native: move rtc to periph

* adapt implementation to new interface
* remove now superfluous drivers directory
native/rtc: clean up documentation
native/rtc: improve implementation

* add error checking / warnings
* implement parts of alarm functionality
* clean up
sys/shell: rewrite rtc command for periph/rtc
examples/default: use periph/rtc instead of rtc

The rtc module is deprecated and will be removed.
FIXUP: val->time
FIXUP: set day of week