kconfig: Sync zconf.y with zconf.tab.c_shipped

Looks like a change to a comment in zconf.y was never committed, because
the updated version only appears it zconf.tab.c_shipped. Update the
comment in zconf.y to match.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>