stm32xx-sys: generate constants for EXTI pins

[hawkw]

Currently, pins used for EXTI IRQs are defined in the sys task's config in app.toml. When a task wishes to actually use those pins, though, it must also define them separately in order to call sys.gpio_configure_input (and if it wants to perform its own GPIO reads on that pin). This is generally done either by hard-coding the port and pin number in the task's source code, or by defining its own config for configuring which pin to use.

This is unfortunate, as it results in a duplicated pin definition, either between the source code and the config if hard-coded, or twice in the config, if the task also gets the definition from config. And, if the task configures the pin in its own config, the author of that task must re-implement the config handling.

This commit fixes this by adding new code generation to drv-stm32xx-sys-api to generate a module called gpio_irq_pins containing PinSet constants with the name of the IRQ pin and the pin number and port defined in the sys task's config section. This way, the same pin set used by sys when configuring the EXTI interrupt can be referenced by the task that consumes the interrupt, removing the need to duplicate the pin definition. I've edited the nucleo-user-button demo task to demonstrate using this.