1) Minor build RP2040 fixes
2) CMake build changes
Examples should be CMake buildable from their own subdirectory; such a build will error out based on
matching .skip.MCUxxx or a mismatched .only.MCU
It should be possible to build from a higher level and use .skip.MCU and .only.MCU to filter which
examples get built
The intention is for the CMakeLists.txts in the examples to be non family specific and without MCU based IFs. I have
started this work, but am not really sure the state of the esp32 stuff; in any case the plan is to have
everything encapsulated in the FAMILY/family.cmake
picoexamples now just includes examples/device/CMakeLists.txt and examples/host/CMakeLists.txt directly, as they
also build correctly when included from there.
Note that .skip.MCU for esp32 in the directories it wasn't previously avaiable has not been added, as the .skip is common to the regular Makefile builds also. It isn't clear whether these examples should build for esp32, but if not .skip should be added.
1) Minor build RP2040 fixes 2) CMake build changes