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: NPeter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: NMichal Marek <mmarek@suse.cz>