Hi Leon,
The current answer for adding that option is yes, you’ll need to clone nerves_system_rpi4 and make a custom system. We’re aware that this is a lot of work for making a one line kernel configuration change.
Having said that, I think that it’s an oversight that the hardware PWM kernel module isn’t being built in the official RPi4 Nerves system. While our systems aim to be minimal, for the Raspberry Pi ones, we have been trying to error on the side of including the basics for attaching hardware devices. If you would like to send a PR to the repository that enables it, that would be appreciated.
There’s also some history on PWM support on the Raspberry Pi. Since the Pi has so few hardware PWM pins, most people (myself included) had either used an I2C->PWM module or used pigpio's DMA controller-based PWM support. pigpio is included in the default Nerves systems for Raspberry Pi specifically for PWM support. If you go the I2C->PWM module route, there are quite a few options. Pololu, Adafruit, and Sparkfun all have modules for them and sometimes have servo connectors if that’s why you’re interested in PWM.
Hope this helps!
