native/rtc: apply naming conventions
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
SQUASHME: Use strtol instead of sscanf
SQUASHME: use plain printf instead of asctime
FIXUP: address comments by @OlegHahm