genrest: Highlight Kconfig as Kconfig, not Python

Sphinx defaults to Python highlighting, but a Kconfig lexer is available
as well
(http://pygments.org/docs/lexers/#pygments.lexers.configs.KconfigLexer

).
Use it.

This only highlights Kconfig definitions without links (references to
other symbols), as Sphinx doesn't highlight '.. parsed-literal::' blocks
with links. It's an improvement over Python stuff getting highlighted at
least.

Side note: '.. highlight:: none' still gives '.. parsed-literal::'
blocks without links a different background color, for whatever reason.

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