kbuild: Fix inconsistent comment

The commit 2042b548 ("kbuild: unset variables in top Makefile
instead of setting 0") renamed the variable from "config-targets"
to "config-build", the comment should be consistent accordingly.

Signed-off-by: Kaiden PK Yu (余泊鎧) <KaidenPK.Yu@moxa.com>
Signed-off-by: SZ Lin (林上智) <sz.lin@moxa.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>