cmake: modules: using posix path for Kconfig generated file

Fixes: #15289

Kconfig requires posix style path when sourcing other files.
As abspath in python will use native host style, i.e. backslash '\' in
windows this will cause invalid paths in Kconfig generated file and
thus the file will never be loaded.

This commit uses PurePath to convert the path to posix style, ensuring
Kconfig can load the modules.

Signed-off-by: Torsten Rasmussen <torsten.rasmussen@nordicsemi.no>