kconfigfunctions.py: Fix used-before-assignment error with pylint 3.2

CI reports error:
kconfigfunctions.py:143:11: E0601: Using variable 'edtlib' before
assignment (used-before-assignment)

Initialize edtlib to none when there is no edt.

Signed-off-by: Ioannis Damigos <ioannis.damigos.uj@renesas.com>