scripts: updated gen_handles.py to take Zephyr base as argument

The script gen_handles.py was introduce in #32127 but relies on
ZEPHYR_BASE being set in environment.

However, it is not a requirement to set Zephyr base in the users
environment, therefore this is changed to an argument `-z` or
`--zephyr-base` which will be passed from the build system to the
script.

If `-z` or `--zephyr-base` is not provided, the environment will be
checked for a ZEPHYR_BASE setting there.

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