PCA9685 Implementation

Following on from my rant about hobbyist software libraries I have now written an initial implementation of a PCA9685 driver from scratch. And you know what? It detects failures and reports them so it’s easier to figure out when and where something is failing!

It’s available in the otb-iot codebase.

Pretty basic right now - just flashes LED 0 connected to device at address 0x40 on and off every second.