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 174a1dcc ("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: 174a1dcc

 ("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>