Answers on a postcard please on how to avoid `static mut` when you have
lazy initialization, and static objects holding references to other
static objects.
Set interrupt enable flags in USB driver. The flags were already being cleared in the USB driver.
Update USB examples to remove interrupt flag set/check as it is no longer necessary.
Implement usb_device for rp2040
Limitations
- RP2040-E5 (Device enumeration workaround) is not implemented
- Suspend/resume is not implemented
- VBus detection is not tested and may not be fully implemented
* Improve clock frequency stuff for uninitialized clocks
- Made clocks singletons so the frequency handling actually works as expected
- Added initial frequencies
- Improved the docs
- Added a Clock trait
* Add pico examples.
These have the benefit of knowing which external crystal is attached.
Even though it always should be a 12 MHz crystal.
Thus we can setup the clocks properly
I also changed the rp2040 examples to work out of the box for pico boards since that will probably be used most of the time
