Make hardware SPI, I2C, ADC optional and add config to optimize for size

[vvuk]

Our good friend STM32F042 has very tiny flash, and I'm stuffing it full of ldc1612 code. There's some hacks I've been doing to get this to fit, and I wanted to turn those into config options. This adds:

• WANT_GPIO_{ADC, SPI, I2C} configs. They depend on their HAVE_GPIO_* base.
• Changes features that depended on HAVE_GPIO_* to WANT_GPIO_*
• Adds a config to optionally compile with -Os instead of -O2

This certainly needs some test cases added at least for compilation, but I want to get this up to see if it would be accepted in general.

Checklist

• pr title makes sense
• squashed to 1 commit
• added a test case if possible
• [-] if new feature, added to the readme
• ci is happy and green