kconfig: Add missing quotes after 'source' and 'menu'

Missing quotes are accepted in some places for compatibility with the C
tools, e.g.

  source no/quotes/Kconfig
  menu Title

It looks broken though. Fix the two places in Zephyr that skip the
quotes. A style warning will be added.

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