scripts: kconfig: kconfigfunctions: use relative ZEPHYR_BASE path

Use a path relative to the script itself to determine the ZEPHYR_BASE path
instead of relying on the environment variable.

This allows for modules to use the Zephyr kconfiglib without having
ZEPHYR_BASE set in the environment.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>