kconfig: Get rid of leading/trailing whitespace in prompts

Leading/trailing whitespace in prompts requires ugly workarounds in
genrest.py, as e.g. *prompt * is invalid RST. strip() all prompts in
Kconfiglib and get rid of the genrest.py workarounds. Add a warning too.

The Kconfiglib update has some unrelated cleanups and fixes (that won't
affect Zephyr).

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>