docs: Fix the docs build with Sphinx 6.0

docs: Fix the docs build with Sphinx 6.0

Sphinx 6.0 removed the execfile_() function, which we use as part of the
configuration process.  They *did* warn us...  Just open-code the
functionality as is done in Sphinx itself.

Tested (using SPHINX_CONF, since this code is only executed with an
alternative config file) on various Sphinx versions from 2.5 through 6.0.

Reported-by: Martin Liška <mliska@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Rebased for U-Boot
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>