kbuild: make -s option take precedence over V=1

'make -s' should be really silent. However, 'make -s V=1' prints noisy
log messages from some shell scripts.

Of course, such a combination is odd, but the build system needs to do
the right thing even if a user gives strange input.

If -s is given, KBUILD_VERBOSE should be forced to 0.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>