Implemented ARCHIVE_OUTPUT_DIRECTORY for uf2 output in CMake. Fixes #1012

[dtreffenstaedt]

This PR enables the use of the ARCHIVE_OUTPUT_DIRECTORY target properties in CMake for the generation of uf2 files in the pico_add_uf2_output function.

The changeset in lines 47..51 is necessary due to CMake seemingly not automatically creating the ARCHIVE_OUTPUT_DIRECTORY if no archive target is present.

I felt the option ARCHIVE_OUTPUT_DIRECTORY is the more appropriate option as opposed to RUNTIME_OUTPUT_DIRECTORY.

If the target property is not set, previous behaviour is preserved.