kconfig: handle comment entries within choice/endchoice

Implement support for comment entries within choice groups. Comment entries
are displayed visually distinct from normal configs, and selecting them is
a no-op.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Cc: Roman Zippel <zippel@linux-m68k.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>