system/nxinit: add CONFIG_SYSTEM_NXINIT_STDOUT for printf-based log output

[JianyuWang0623]

Summary

system/nxinit currently logs everything through syslog(). On
products where syslog is routed through a userspace service (e.g.
DFX), that channel may not be usable yet during early boot, or may
not be visible at all if the service itself is what's failing (e.g.
a service start failure, or an exception inside the DFX consumer).
When that happens, init_debug/info/warn/err output is silently
lost and there is no way to see why init failed to bring up a
service.

Add a debug-only Kconfig option SYSTEM_NXINIT_STDOUT that redirects
all four log macros to printf() (with an appended newline) instead
of syslog(), selected at compile time via a new init_log_output()
macro. Default is n (unchanged syslog behavior).

Impact

• Only touches system/nxinit/{Kconfig,init.h}. No behavior change
  when SYSTEM_NXINIT_STDOUT is left at its default (n).
• No new dependencies; stdio.h is already transitively available.

Testing

Built with the sim:nsh target (host gcc, nxinit-qemu-sim-all-nsh-defconfigs
branch of nuttx which enables CONFIG_SYSTEM_NXINIT). nxstyle
clean on both touched files.

Verified the actual failure mode this fixes: pointed
CONFIG_SYSTEM_NXINIT_RC_FILE_PATH at a nonexistent file so init
hits its own init_err() path on boot, and disabled
CONFIG_SYSLOG_DEFAULT so syslog() has no console channel at all
(simulating "syslog not available/not visible"):

# CONFIG_SYSTEM_NXINIT_STDOUT is not set, CONFIG_SYSLOG_DEFAULT is not set
$ ./nuttx
nsh>                     # no init_err output at all - the bug this fixes

# CONFIG_SYSTEM_NXINIT_STDOUT=y, CONFIG_SYSLOG_DEFAULT still not set
$ ./nuttx
Error Opening /etc/init.d/init.rc.MISSING 2
nsh>                     # init_err output now visible via printf

With CONFIG_SYSTEM_NXINIT_STDOUT left unset (default), behavior is
unchanged from before this patch (verified boot log is identical to
apache/master).