doc: _extensions: kconfig: allow overwriting Kconfig path

Allow specifying the Kconfig file path in the environment (in practice
in the `conf.py` file), to allow the script to correctly find Kconfig
roots that aren't in the expected `"modules" / name / "Kconfig"`
location.

This is required since in the normal build system, the
`ZEPHYR_{name_var}_KCONFIG` cmake symbol can be constructed arbitrarily
in `modules.cmake`.

Signed-off-by: Jordan Yates <jordan@embeint.com>