kbuild: make -s should be used with kernelrelease/kernelversion/image_name

If .config has been edited, there will be a silentoldconfig run:

  $ make defconfig
  ...
  $ make kernelrelease
  scripts/kconfig/conf --silentoldconfig Kconfig
  3.16.0-rc1+

Recently, kbuild started to print the name of the build directory when
using O=

  $ make O=build kernelrelease
  make[1]: Entering directory `/dev/shm/mmarek/linux-2.6/build'
  3.16.0-rc1+

Since these targets are often used in scripts, add a hint to use make -s
to the help text.
Suggested-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -1214,9 +1214,9 @@ help:
 	@echo  '  tags/TAGS	  - Generate tags file for editors'
 	@echo  '  cscope	  - Generate cscope index'
 	@echo  '  gtags           - Generate GNU GLOBAL index'
-	@echo  '  kernelrelease	  - Output the release version string'
-	@echo  '  kernelversion	  - Output the version stored in Makefile'
-	@echo  '  image_name	  - Output the image name'
+	@echo  '  kernelrelease	  - Output the release version string (use with make -s)'
+	@echo  '  kernelversion	  - Output the version stored in Makefile (use with make -s)'
+	@echo  '  image_name	  - Output the image name (use with make -s)'
 	@echo  '  headers_install - Install sanitised kernel headers to INSTALL_HDR_PATH'; \
 	 echo  '                    (default: $(INSTALL_HDR_PATH))'; \
 	 echo  ''