Host GPIO improvements

[rhulme]

The host platform GPIO code has a few issues:

• Only one function has been declared 'weak' so any project wishing to implement a more concrete GPIO simulation has to disable that part of the SDK (by setting SKIP_HARDWARE_GPIO to 1 before importing the SDK) and provide it's own implementation of the full interface.
• The function parameters are not marked as 'unused' so generate lots of warnings if a higher warning level has been set.
• Many functions from rp2_common are missing