Initializing I2C RTC (DS3231) on Raspberry PI bootup with Systemd without recompiling the kernel or devicetree support.

rtc-i2c

# /etc/conf.d/rtc-i2c
#
# My chip is actually a ds3231n, but ds1307 driver works fine (ds3232 does not!)
#
CHIP="ds1307"
ADDRESS="0x68"
BUS="1"

rtc-i2c.conf

# /etc/modules-load.d/rtc-i2c.conf
i2c-dev
i2c_bcm2708

rtc-i2c.rules

# /etc/udev/rules.d/rtc-i2c.rules
#
# I would prefer to put SUBSYSTEMS="i2c",
# but for some reason it's not working on my system.
#
ACTION=="add", SUBSYSTEM="rtc", ATTRS{hctosys}=="0", RUN+="/usr/bin/hwclock -s --utc"

rtc-i2c.service

# /lib/systemd/system/rtc-i2c.service

[Unit]
Description=Initialize i2c hardware RTC device driver
DefaultDependencies=no
Requires=systemd-modules-load.service
After=systemd-modules-load.service
Before=sysvinit.target
ConditionPathExists=/sys/class/i2c-adapter
Conflicts=shutdown.target

[Service]
Type=oneshot
RemainAfterExit=yes
EnvironmentFile=/etc/conf.d/rtc-i2c
ExecStart=/bin/sh -c "echo ${CHIP} ${ADDRESS} > /sys/class/i2c-adapter/i2c-${BUS}/new_device"

[Install]
WantedBy=sysinit.target