Develop code structure to support many target architectures

[susanw1]

It's hard to build a source-tree where mostly the same code supports many hardware devices. Eg we are targeting 3 ARM MCUs at the moment, but there are actually >1000 distinct STM32 devices with different generations of different sub-systems in each. But the thing we want to produce is a working build for a user-selected specific device. How to handle this?

Goals:

• Confine the number of build-modules (previous attempt needed >1 build-module per target device, so avoid that - too fine grain)
• Simple include paths
• Supports multiple low-level interfaces (eg ZLL, Arduino, etc) for a given peripheral
• Supports multiple implementations of those interfaces (eg polling vs interrupt-driven vs DMA)
• Supports interaction between peripherals without coupling them together
• Allows user-level config to specify pins, DMAs etc
• Gives (meaningful) compile error on invalid setups

Options for how the device source-tree should be segmented:

1. One build-module overall - single source-tree (!). Simple, structure, hard to find things, any change re-releases everything;
2. One module per manufacturer (STM32, Texas etc). Keeps the wildly different low-level code in different places.
3. One module per interface (ZLL, Arduino, etc)
4. Combination: One source-tree per interface/manufacturer.

After discussion, we like option#4. Something like "Arduino" is built on the Arduino HAL, which hides device detail. But the Zscript ZLL implementation needs to be divided by manufacturer.