x86/build: Silence the build with "make -s"

Every kernel build on x86 will result in some output:

  Setup is 13084 bytes (padded to 13312 bytes).
  System is 4833 kB
  CRC 6d35fa35
  Kernel: arch/x86/boot/bzImage is ready  (#2)

This shuts it up, so that 'make -s' is truely silent as long as
everything works. Building without '-s' should produce unchanged
output.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170719125310.2487451-6-arnd@arndb.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/Makefile

@@ -73,12 +73,13 @@ UBSAN_SANITIZE := n
 $(obj)/bzImage: asflags-y  := $(SVGA_MODE)
 
 quiet_cmd_image = BUILD   $@
+silent_redirect_image = >/dev/null
 cmd_image = $(obj)/tools/build $(obj)/setup.bin $(obj)/vmlinux.bin \
-			       $(obj)/zoffset.h $@
+			       $(obj)/zoffset.h $@ $($(quiet)redirect_image)
 
 $(obj)/bzImage: $(obj)/setup.bin $(obj)/vmlinux.bin $(obj)/tools/build FORCE
 	$(call if_changed,image)
-	@echo 'Kernel: $@ is ready' ' (#'`cat .version`')'
+	@$(kecho) 'Kernel: $@ is ready' ' (#'`cat .version`')'
 
 OBJCOPYFLAGS_vmlinux.bin := -O binary -R .note -R .comment -S
 $(obj)/vmlinux.bin: $(obj)/compressed/vmlinux FORCE