doc: fix sphinx 1.5 broken search box

Sphinx 1.5 introduced changes that broke the
sphinx-supplied search (as explained in
https://github.com/sphinx-doc/sphinx/pull/2454)

This patch, along with a patch in the docs-theme
https://github.com/zephyrproject-rtos/docs-theme/pull/10


repairs things.

Jira: INF-136

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>