system/nxinit: improve init.rc parser (per-cpu configs, --option fix)

[JianyuWang0623]

Note: Please adhere to Contributing Guidelines.

Summary

This series improves the system/nxinit init.rc parser. It contains two
self-contained, independent commits:

1. parse default cpu-specific configs — On top of the configured rc file
   (CONFIG_SYSTEM_NXINIT_RC_FILE_PATH, e.g. /etc/init.d/init.rc),
   init_parse_configs() now also loads an optional per-cpu config derived
   from that path (<rc-path>.cpu${CPUID}, e.g. /etc/init.d/init.cpu0.rc).
   The configurable rc path support is preserved and the unused directory
   scanning path is dropped.

2. fix argument parser treating --option as -- separator —
   init_parse_arguments() only compared the first two characters when
   detecting the -- separator, so options like --system / --nofork were
   mistaken for the standalone -- separator and the remaining arguments were
   truncated. Add an isblank() check on the third character so only a
   standalone -- followed by whitespace triggers the separator logic.

Both commits are independent parser improvements and can be reviewed in any
order.

Impact

• Users: nxinit gains optional per-cpu init.rc support and correctly
  passes through long options (e.g. --system) to services/actions. Existing
  single-file init.rc setups are unaffected (the per-cpu file is optional).
• Build: No new Kconfig options; no build-flag changes. Only
  system/nxinit/{parser.c,parser.h,init.c} are touched.
• Hardware / compatibility: No ABI or on-flash format change. Behavior for
  configs that do not use per-cpu files or ---style options is unchanged.
• Documentation / security: None.

Testing

Host:

• OS: Linux (x86_64)
• Compiler: xtensa-esp32s3-elf-gcc (ESP toolchain)

Target:

• arch: xtensa
• board:config: lckfb-szpi-esp32s3:adb

Verification:

• ./tools/checkpatch.sh -f passes cleanly on all touched files
  (parser.c, parser.h, init.c) — All checks pass.
• The full nxinit series (of which this is the first batch) builds and boots on
  the target above; init.rc is parsed and services start as expected.