* Added ARCHIVE_OUTPUT_DIRECTORY to uf2 output in CMake
This Commit enables the use of the ARCHIVE_OUTPUT_DIRECTORY target
properties in CMake for the generation of uf2 files.
The changeset in lines 47..51 is necessary due to CMake not
automatically creating the ARCHIVE_OUTPUT_DIRECTORY if no archive target
is present.
* rework cmake changes to make it friendler for non absolute paths
Co-authored-by: Graham Sanderson <graham.sanderson@raspberrypi.com>