nconf: handle comment entries within choice/endchoice

Equivalent to af6c1598 (kconfig: handle comment entries within
choice/endchoice), but for nconfig instead.

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>
Signed-off-by: Michal Marek <mmarek@suse.cz>