doc: do not use env vars to discover paths

ZEPHYR_BASE and ZEPHYR_BUILD directories were passed to Sphinx via
environment variables. However, these paths can be easily discovered by
the conf.py file itself. As a result, Sphinx is less dependent on CMake
to operate.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
2 files changed