[5.14-rc1] kbuild: stdout from cmd should not be discarded

[Tetsuo Handa]

Commit 174a1dcc96429efc ("kbuild: sink stdout from cmd for silent build")
confuses users by sending prompt messages from

  scripts/kconfig/conf -s --syncconfig

to /dev/null before start waiting for response to prompt messages
when building with "make -s". Reverting that commit fixes this problem.

----------
$ make
  SYNC    include/config/auto.conf.cmd
*
* Restart config...
*
*
* General setup
*
Compile also drivers which will not load (COMPILE_TEST) [N/y/?] n
Local version - append to kernel release (LOCALVERSION) []
Automatically append version information to the version string (LOCALVERSION_AUTO) [N/y/?] n
Build ID Salt (BUILD_SALT) []
Kernel compression mode
> 1. Gzip (KERNEL_GZIP)
  2. Bzip2 (KERNEL_BZIP2)
  3. LZMA (KERNEL_LZMA)
  4. XZ (KERNEL_XZ)
  5. LZO (KERNEL_LZO)
  6. LZ4 (KERNEL_LZ4)
  7. ZSTD (KERNEL_ZSTD)
choice[1-7?]: 1
Default init path (DEFAULT_INIT) []
Default hostname (DEFAULT_HOSTNAME) [(none)] (none)
Support for paging of anonymous memory (swap) (SWAP) [Y/n/?] y
System V IPC (SYSVIPC) [Y/n/?] y
POSIX Message Queues (POSIX_MQUEUE) [Y/n/?] y
General notification queue (WATCH_QUEUE) [Y/n/?] y
Enable process_vm_readv/writev syscalls (CROSS_MEMORY_ATTACH) [Y/n/?] y
uselib syscall (USELIB) [Y/n/?] y
Auditing support (AUDIT) [Y/n/?] y
Preemption Model
  1. No Forced Preemption (Server) (PREEMPT_NONE)
  2. Voluntary Kernel Preemption (Desktop) (PREEMPT_VOLUNTARY)
> 3. Preemptible Kernel (Low-Latency Desktop) (PREEMPT)
choice[1-3?]: 3
Core Scheduling for SMT (SCHED_CORE) [N/y/?] (NEW) ^Cmake[2]: *** [scripts/kconfig/Makefile:77: syncconfig] Interrupt
make[1]: *** [Makefile:626: syncconfig] Interrupt
make: *** [Makefile:735: include/config/auto.conf.cmd] Interrupt