kconfig: highlight xconfig 'comment' lines with '***'

Mark Kconfig "comment" lines with "*** <commentstring> ***"
so that it is clear that these lines are comments and not some
kconfig item that cannot be modified.

This is helpful in some menus to be able to provide a menu
"sub-heading" for groups of similar config items.

This also makes the comments be presented in a way that is
similar to menuconfig and nconfig.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>