When cloning the pico-sdk repo manually, one normally would do `git
submodule update --init`, which is non-recursive. However, when cloning
automatically, CMake will recursively update submodules by default.
Updating all of tiny-usb's submodules takes an extremely long time.
Luckily, CMake 3.17 added an option we can specify for FetchContent to
tell it not to recursively update submodules. On older CMake versions,
the flag is not used. For those with a new enough version of CMake, this
will significantly speed up SDK cloning.
Fixes#771.
