kbuild: do not suppress Kconfig prompts for silent build

When a new CONFIG option is available, Kbuild shows a prompt to get
the user input.

  $ make
  [ snip ]
  Core Scheduling for SMT (SCHED_CORE) [N/y/?] (NEW)

This is the only interactive place in the build process.

Commit 174a1dcc96 ("kbuild: sink stdout from cmd for silent build")
suppressed Kconfig prompts as well because syncconfig is invoked by
the 'cmd' macro. You cannot notice the fact that Kconfig is waiting
for the user input.

Use 'kecho' to show the equivalent short log without suppressing stdout
from sub-make.

Fixes: 174a1dcc96 ("kbuild: sink stdout from cmd for silent build")
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

Makefile

@@ -728,11 +728,12 @@ $(KCONFIG_CONFIG):
 # This exploits the 'multi-target pattern rule' trick.
 # The syncconfig should be executed only once to make all the targets.
 # (Note: use the grouped target '&:' when we bump to GNU Make 4.3)
-quiet_cmd_syncconfig = SYNC    $@
-      cmd_syncconfig = $(MAKE) -f $(srctree)/Makefile syncconfig
-
+#
+# Do not use $(call cmd,...) here. That would suppress prompts from syncconfig,
+# so you cannot notice that Kconfig is waiting for the user input.
 %/config/auto.conf %/config/auto.conf.cmd %/generated/autoconf.h: $(KCONFIG_CONFIG)
-	+$(call cmd,syncconfig)
+	$(Q)$(kecho) "  SYNC    $@"
+	$(Q)$(MAKE) -f $(srctree)/Makefile syncconfig
 else # !may-sync-config
 # External modules and some install targets need include/generated/autoconf.h
 # and include/config/auto.conf but do not care if they are up-to-date.