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>