* Add boot2 feature. Add boot2 linkage into each BSP optional on feature
* Enable boot2 feature in BSPs by default. Remove boot2 decl from all BSP examples
* Add EXTERN in memory.x for BOOT2_FIRMWARE, rename bootloader static slice to BOOT2_FIRMWARE
* Update new examples and itsy_bitsy BSP to use boot2 feature
* Remove boot2 as a dev-dependency for the BSPs, no longer needed
* Add no_mangle BOOT2_FIRMWARE to adafruit_macropad
* Fix itsy-bitsy blinky - it wasn't using the BSP, so it didn't get BOOT2_FIRMWARE linked in
We updated the behavior of boot2 so that there is now no default bootloader.
Pico_explorer_showcase was the only BSP still using git:main, pin
it to the last git commit before https://github.com/rp-rs/rp2040-boot2-rs/pull/7
* 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
