kbuild: suppress stdout from merge_config for silent builds

kbuild: suppress stdout from merge_config for silent builds

merge_config does not respect the Make's -s (--silent) option.

Let's sink the stdout from merge_config for silent builds.

This commit does not cater to the direct invocation of merge_config.sh
(e.g. arch/mips/Makefile).

Reported-by: Leon Romanovsky <leon@kernel.org>
Closes: https://lore.kernel.org/all/e534ce33b0e1060eb85ece8429810f087b034c88.1733234008.git.leonro@nvidia.com/
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>