* miscellaneous cleanup:
* cleanup some #ifdefs which were slightly hacky when RP2350 was added; use HAS_ flags in preference to PICO_RP2040/RP2350
* make some dependencies more explicit - i.e. compile if the user doesn't want to include certain libraries
* cleanup some directory A -> directory B relative path names in CMakeLists.txt to be SDK root -> directory B
* Remove deprecated uses of FetchContent_Populate
Cannot use MakeAvailable as the files shouldn't be added to the build, so instead uses the full signature for FetchContent_Populate
Fixes#2124
* Fix GIT_SUBMODULES_RECURSE for older CMake versions
