system/nxinit: add rptun/unlink builtins and service fallback option

[JianyuWang0623]

Summary

1. rptun / unlink builtin commands. Some products drive early bring-up
   entirely from init.rc rather than NSH, so the NSH rptun command is
   not available there. Add rptun as an init builtin (guarded by
   CONFIG_RPTUN) supporting start/stop, plus a generic unlink
   builtin (always available) for removing device nodes, e.g.:

   rptun stop /dev/rptun/proc
   unlink /dev/rptun/proc
   rptun start /dev/rptun/proc
   

   RPTUNIOC_START returns the pid of the rptun kernel thread in async
   mode (CONFIG_RPTUN_START_SYNC unset). That thread is a detached
   kthread, never a child of init, so returning a positive value would
   make the action engine waitpid() on it and block the whole action
   queue (and on init / console would never run). A successful start is
   normalized to 0.

2. fallback service option. Add an SVC_FALLBACK flag and a
   fallback service option — the semantic opposite of override — to
   resolve same-name service conflicts. override makes the new
   definition replace the old; fallback makes the definition marked
   fallback yield to the other one, so a board-level init.rc can provide
   a default service that is silently dropped when another init.rc
   defines a service with the same name (and vice versa). If neither flag
   is set, duplicate service names still produce -EEXIST as before.

Impact

• Only touches system/nxinit/{builtin.c,service.c,service.h}. No change
  to existing builtins/options; new behavior is opt-in via the new
  rptun/unlink commands and the fallback option.
• rptun builtin is compiled only when CONFIG_RPTUN is enabled;
  unlink and fallback are always available.
• No new Kconfig symbols.

Testing

Built and verified locally with the sim target (host gcc). nxstyle
clean on all three touched files.

fallback verified at runtime on sim + nxinit with an init.rc that
declares two services named console (A/B against the same config
without the flag):

# without "fallback":
Error Redefined service 'console'          # parse fails (-EEXIST)

# with "fallback" on the second definition:
nsh> ps
  TID  PID PPID ... COMMAND
    4    4    0 ... init_main               # nxinit is init
    5    5    4 ... sh                       # the non-fallback console starts;
                                             # the fallback one is silently ignored, no error

unlink is always compiled in and rptun compiles under CONFIG_RPTUN;
both build cleanly. The sim target has no rptun device, so the rptun
runtime path is not exercised here — the concurrency fix (normalizing a
successful async RPTUNIOC_START to 0 so the action engine does not
waitpid() on the detached rptun kthread) is a targeted one-line change
verified by build + code review.

cc @wyr-7

[JianyuWang0623]

Note: The Assisted-by trailer appears only in commits that were originally authored with the help of AI coding tools. Some commits in this series (e.g., the rptun/unlink builtin and the fallback service option) were originally created on our internal Gerrit at an earlier date, before AI-assisted workflows were adopted, so they do not carry the Assisted-by tag. The commit message content is otherwise preserved as-is from the original internal commits.