Makefile: add comment to discourage tools/* addition for kernel builds

Kbuild provides scripts/Makefile.host to build host programs used for
building the kernel. Unfortunately, there are two exceptions that opt
out of Kbuild. The build system under tools/ is a cheesy replica, and
cause issues. I was recently poked about a problem in the tools build
system, which I do not maintain (and nobody maintains). [1]

Without a comment, people might believe this is the right location
because that is where objtool lives, even if a more robust Kbuild
syntax satisfies their needs. [2]

[1]: https://lore.kernel.org/linux-kbuild/ZnIYWBgrJ-IJtqK8@google.com/T/#m8ece130dd0e23c6f2395ed89070161948dee8457
[2]: https://lore.kernel.org/all/20240618200501.GA1611012@google.com/



Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <nicolas@fjasle.eu>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>