feat(nrf52): optional external QSPI LittleFS filesystem

[benallfree]

This PR creates a LittleFS partition on external flash for nRF devices that support it and exposes a new extFs global to access it. We introduce a new flag, MESHTASTIC_EXTERNAL_FLASH_FS that will initialize a LittleFS partition on external flash.

Co-author: @GoatsAndMonkeys

Prerequisites:

• merge fix(xmodem): truncate file on open instead of appending #10143

Note that if MESHTASTIC_EXTERNAL_FLASH_FS is defined but EXTERNAL_FLASH_USE_QSPI is not, it is assumed that external flash is not supported and thus this all becomes a noop. Both must be defined for this feature to work.

Boards with external flash hard-wired from factory

The following boards have MESHTASTIC_EXTERNAL_FLASH_FS set by default in this PR because
their QSPI flash chip is physically soldered on every unit shipped:

Board env	Product	Chip
t-echo	Lilygo T-Echo	WP25R1635F / MX25R1635F (2 MB)
t-echo-lite	Lilygo T-Echo Lite	ZD25WQ32CEIGR (4 MB)
t-echo-plus	Lilygo T-Echo Plus	MX25R1635F (2 MB)
nano-g2-ultra	B&Q Consulting Nano G2 Ultra	W25Q16JV (2 MB)
rak_wismeshtap	RAK WisMesh TAP	IS25LP080D (1 MB)

Boards with QSPI capability but NOT enabled in this PR

The following boards define EXTERNAL_FLASH_USE_QSPI and name a chip, but the chip's presence on every shipped unit could not be confirmed from public hardware documentation (modular ecosystems, development kits, community boards). Board maintainers should add #define MESHTASTIC_EXTERNAL_FLASH_FS to their variant.h once confirmed:

Board env	Notes
rak4631 and all rak4631_* variants	WisBlock modular — flash is on the base board, not the core module; not all base board configurations include it
rak3401_1watt	RAK module variant — flash presence depends on carrier board
heltec_mesh_node_t114	Heltec — likely soldered but not verified
canaryone	Community board — not verified
meshlink	Community board — not verified
meshtiny	Community board — not verified
monteops_hw1	Community board — not verified
muzi_base	Community board — not verified
gat562_mesh_trial_tracker	Custom tracker — not verified
seeed_wio_tracker_L1 / _eink	Seeed — likely soldered but not verified
seeed_solar_node	Seeed — not verified
seeed_xiao_nrf52840_kit	XIAO kit — flash may be kit accessory only
ELECROW-ThinkNode-M1 / M6	Elecrow — not verified

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other (please specify below)