Enable i2c RTC using systemd.
A udev rules triggers an I2C systemd service on I2C kernel module adding.
Service then loads I2C driver adding a new I2C device to /sys
Then kernel loads I2C RTC driver and adds an RTC device
It triggers a udev rules that updates hardware clock
- enable I2C in config.txt, adding
device_tree_param=i2c1=on