add feature rpirtc
Add new feature rpi rtc allowing a hw rtc to be used. The init script in Alpine Linux since v3.9 looks for /dev/rtc, if not found it will switch to swclock. To make this check work on a Rasberry PI with a mounted rtc and the following in usercfg.txt dtoverlay=i2c-rtc,ds3231 we must have rtc drivers available already initramfs.
rpirtcfeature in mkinitfs is (was) used: https://community.riocities.com/alpine_rpi_rtc.html#setup