xtensa-build-zephyr.py: cleanup and re-organize to prepare for installing more files

[marc-hb]

This is preparing the addition of zephyr.elf, zephyr.lst and possibly other files to build-sof-staging. For now this PR should make no difference, the purpose is to make the actual addition much smaller and easier to review and to find any bugs in this one before the actual change.

No difference except maybe one small bugfix in the final "tree" listing - depending on whether pathlib.glob(**/*) was ever random and where.

This also makes pylint xtensa-build-zephyr.py finally usable.