Prefer tmpfs for /tmp mount in SIM and update documentation

[arjav1528]

Note: Please adhere to Contributing Guidelines.

Fixes #12791

Why this change is needed

• Current SIM rc.sysinit behavior always creates a FAT RAMDISK and mounts it on /tmp.
• NuttX already supports TMPFS, and expected behavior is to use TMPFS for /tmp when available.

What this change does

• Updates boards/sim/sim/sim/src/etc/init.d/rc.sysinit to:
  • Mount /tmp as tmpfs when CONFIG_FS_TMPFS=y.
  • Fall back to existing FAT RAMDISK flow when TMPFS is not enabled and CONFIG_FS_FAT=y.
• Updates sched/Kconfig help text for ETC_ROMFS and FAT options so documentation matches implementation.
• Updates documentation to reflect TMPFS-first /tmp behavior with FAT fallback:
  • Documentation/guides/etcromfs.rst
  • Documentation/applications/nsh/customizing.rst

Impact

• Users: SIM configurations with TMPFS enabled now get /tmp backed by TMPFS by default.
• Compatibility: FAT RAMDISK fallback is preserved for configurations without TMPFS.
• Build process: No build system changes.
• Hardware: Change is scoped to SIM startup script behavior.
• Documentation: Kconfig help and guides now align with runtime behavior.
• Security: No direct security impact identified.

What was tested

1. File-level style checks on all touched files:

./tools/checkpatch.sh -f boards/sim/sim/sim/src/etc/init.d/rc.sysinit \
  sched/Kconfig \
  Documentation/guides/etcromfs.rst \
  Documentation/applications/nsh/customizing.rst

[acassis]

Thank you @arjav1528