system/nxinit: increase SERVICE_ARGS_MAX default and add 'set' builtin

[JianyuWang0623]

Summary

Two independent, low-risk fixes carried over from the internal (vela) fork of this component:

1. Increase SYSTEM_NXINIT_SERVICE_ARGS_MAX default from 8 to 16. The previous default is insufficient for services with many arguments (e.g. ptpd needs 10 argv slots). When exceeded, argv lacks a NULL terminator, causing posix_spawnp to read out of bounds.
2. Add a set KEY VALUE builtin command. Previously set in init.rc fell through to posix_spawnp(), which ran it in a temporary child shell — the environment variable was set only in that child and lost when it exited, so later services never inherited it. cmd_set now calls setenv() directly in the init process, guarded by CONFIG_DISABLE_ENVIRON, so child processes (which inherit init's environment) see it correctly.

Impact

• system/nxinit/Kconfig: change default range value, no new config surface.
• system/nxinit/builtin.c: add one builtin command entry, no changes to existing command behavior.

Testing

Built for esp32p4-function-ev-board:nsh (CONFIG_SYSTEM_NXINIT=y) with the actual toolchain (riscv-none-elf-gcc):

$ make -j$(nproc)
...
LD:  nuttx
MKIMAGE: NuttX binary
Generated: nuttx.bin

Verified via nm nuttx that cmd_set is linked into the image, and confirmed CONFIG_SYSTEM_NXINIT_SERVICE_ARGS_MAX=16 takes effect in the generated .config.

Ran nxstyle on the changed file (builtin.c) with no violations.

Runtime verification (esp32p4-pico-wifi-wareshare, MAC 80:f1:b2:d0:84:c4, /dev/ttyACM3)

The lckfb-szpi-esp32s3 board normally used for hardware verification was not reachable on any /dev/ttyACM*//dev/ttyUSB* this time (esptool.py read_mac failed on all of them, likely not connected). Both changes in this PR live in apps/system/nxinit and are board-agnostic, so an esp32p4 board was used as an equivalent target.

Flashed the branch build and captured the nsh serial console:

$ esptool.py -c esp32p4 -p /dev/ttyACM3 -b 921600 write_flash 0x2000 nuttx.bin
...
Wrote 281368 bytes (139619 compressed) at 0x00002000 in 3.1 seconds...
Hash of data verified.
Hard resetting via RTS pin...

nsh> ps
  TID   PID  PPID PRI POLICY   TYPE    NPX STATE    EVENT     SIGMASK            STACK COMMAND
    0     0     0   0 FIFO     Kthread   - Ready              0000000000000000 0002016 Idle_Task
    2     2     0 100 RR       Task      - Waiting  Semaphore 0000000000000000 0008120 init_main
    3     3     2 100 RR       Task      - Running            0000000000000000 0008144 sh

init_main (nxinit, PID 2) is the parent of sh (PID 3), confirming nxinit is functioning correctly as system init with CONFIG_SYSTEM_NXINIT_SERVICE_ARGS_MAX=16 in the running firmware.

To exercise the new set builtin specifically, a temporary set NXINIT_SET_TEST hello_from_set_builtin line was added to init.rc's on init block (build/flash/verify only — reverted afterward, not part of this PR):

nsh> env
PWD=/
NXINIT_SET_TEST=hello_from_set_builtin

This confirms set KEY VALUE is parsed and executed from init.rc, setenv() takes effect in the init process, and the spawned child (sh) inherits the variable. After this check, the init.rc change was reverted (git status clean on both nuttx and apps repos) — no code in this PR was altered.