kbuild: Correctly deal with make options which contain an "s"

When using remake, which is based on gnumake, if you invoke
an example build as shown below, the build will become silent
due to the top level make file incorrectly guessing that
the end user wants a silent build because an argument that
contained an "s" was used.  Here are two examples one with remake
and one with straight gnumake.

remake --no-extended-errors
make --warn-undefined-variables

Fix up the top level Makefile to use filter to parse the options
that mean silent instead of findstring catching other random
arguments containing an "s".

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
CC: Michal Marek <mmarek@suse.cz>
CC: Andrew Morton <akpm@linux-foundation.org>
CC: linux-kbuild@vger.kernel.org
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -312,7 +312,7 @@ endif
 # If the user is running make -s (silent mode), suppress echoing of
 # commands
 
-ifneq ($(findstring s,$(MAKEFLAGS)),)
+ifneq ($(filter s% -s%,$(MAKEFLAGS)),)
   quiet=silent_
 endif