Makefile: Fix lying comment re. silentoldconfig

The comment above the silentoldconfig invocation is outdated.
'make oldconfig' updates just .config and doesn't touch the
include/config/ tree.

This came up in https://lkml.org/lkml/2018/2/12/415

.

While fixing the comment, make it more informative by explaining the
purpose of the unfortunately named silentoldconfig.

I can't make sense of the comment re. auto.conf.cmd and a cleaned tree.
include/config/auto.conf and include/config/auto.conf.cmd are both
created simultaneously by silentoldconfig (in
scripts/kconfig/confdata.c, by conf_write_autoconf()), and nothing seems
to remove auto.conf.cmd that wouldn't remove auto.conf. Remove that part
of the comment rather than blindly copying it. It might be a leftover
from an older way of doing things.

The include/config/auto.conf.cmd prerequisite might be there to ensure
that silentoldconfig gets rerun if conf_write_autoconf() fails between
writing out auto.conf.cmd and auto.conf (a comment in the function
indicates that auto.conf is deliberately written out last to mark
completion of the operation). It seems the Makefile dependency between
include/config/auto.conf and .config would already take care of that
though, since include/config/auto.conf would still be out of date re.
.config if the operation fails.

Cop out and leave the prerequisite in for now.

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