kconfig: make comments stand out in menuconfig

Matěj Laitl <strohel@gmail.com> noticed that there was no way
to distingush between comments and un-selectable menu lines.
This patch marks comments with *** comment ***

Cc: Matěj Laitl <strohel@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>