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.

Makefile

@@ -43,6 +43,7 @@ CONF_FILES	:= mkinitfs.conf \
 		features.d/raid.files \
 		features.d/raid.modules \
 		features.d/reiserfs.modules \
+		features.d/rpirtc.modules \
 		features.d/scsi.modules \
 		features.d/squashfs.modules \
 		features.d/ubifs.modules \

features.d/rpirtc.modules

+kernel/drivers/rtc/rtc-ds1307.ko

[Henrik Riomar]

How the rpirtc feature in mkinitfs is (was) used: https://community.riocities.com/alpine_rpi_rtc.html#setup