scripts: ensuring posix path on module defined roots.

Fixes: #29235

This commit ensures that roots are being converted to posix paths.
This fixes the issue where windows style path (containing `\`) would
result in DTS dependency file to contain mixed style path separator and
thus causing Ninja to re-invoke CMake in an endless loop.

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