docs/extra-build-config.md: Document CAN_OSCILLATOR variable
docs/extra-build-config.md: Document CAN_OSCILLATOR variable

Add instructions how to use CAN_OSCILLATOR variable and declare support
for the Waveshare RS485 CAN HAT.

Signed-off-by: Jakub Luzny <1d3764b91b902f6b45836e2498da81fe35caf6d6@luzny.cz>

agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi