kbuild: suppress annoying "... is up to date." message

kbuild: suppress annoying "... is up to date." message

Under certain conditions, Kbuild shows "... is up to date" where
if_changed or friends are used.

For example, the incremental build of ARM64 Linux shows this message
when the kernel image has not been updated.

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-
    CHK     include/config/kernel.release
    CHK     include/generated/uapi/linux/version.h
    CHK     include/generated/utsrelease.h
    CHK     include/generated/bounds.h
    CHK     include/generated/timeconst.h
    CHK     include/generated/asm-offsets.h
    CALL    scripts/checksyscalls.sh
    CHK     include/generated/compile.h
    CHK     kernel/config_data.h
  make[1]: `arch/arm64/boot/Image.gz' is up to date.
    Building modules, stage 2.
    MODPOST 0 modules

The following is the build rule in arch/arm64/boot/Makefile:

  $(obj)/Image.gz: $(obj)/Image FORCE
          $(call if_changed,gzip)

If the Image.gz is newer than the Image and the command line has not
changed (i.e., $(any-prereq) and $(arg-check) are both empty), the
build rule $(call if_changed,gzip) is evaluated to be empty, then
GNU Make reports the target is up to date.  In order to make GNU Make
quiet, we need to give it something to do, for example, "@:".  This
should be fixed in the Kbuild core part rather than in each Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Marek <mmarek@suse.com>