Check for a symbol first in _finalize_tree(), and refactor a bit
Check for a symbol first in _finalize_tree(), and refactor a bit

Symbol is the most common case by far.

Also rename _finalize_tree() to _finalize_node(), and make the
explanation for what it does more specific. The recursion gets a bit
tricky.

ulfalizer/Kconfiglib,ulfalizer/Kconfiglib