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: NMasahiro Yamada <masahiroy@kernel.org>

Makefile

@@ -96,6 +96,7 @@ endif
 
 ifneq ($(findstring s,$(filter-out --%,$(MAKEFLAGS))),)
   quiet=silent_
+  KBUILD_VERBOSE = 0
 endif
 
 export quiet Q KBUILD_VERBOSE