XIAO BLE cleanup (supporting changes to seeed_xiao_nrf52840_kit too)

[ndoo]

• seeed_xiao_nrf52840_kit: Use build flag for L76K GNSS, rename variant.h ifdef
• seeed_xiao_nrf52840_kit: Support multiple SX126x pinouts via build flags
• seeed_xiao_nrf52840_kit: Pin D0 as user button if pin is unused
• EBYTE E22 and NiceRF gain and SX1262 max power defines
• xiao_ble: Move variant to DIY and extend from seeed_xiao_nrf52840_kit
• seeed_xiao_nrf52840_kit: Pin D6, D7 as I2C SDA, SCL if pins are unused

🤝 Attestations

• [x] 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
  • [x] Other (please specify below)
    • [x] xiao_ble
    • [x] seeed_xiao_nrf52840_kit

[ndoo]

This patchset supports further DIY XIAO nRF52840 variants, such as: https://github.com/ndoo/meshtastic-firmware/compare/xiao-diy-cleanup...ndoo:meshtastic-firmware:xiao-nrf52840-e22

[ndoo]

Some feedback or next steps would be welcome, because I am pretty much stuck on the way forward for DIY builds based on these boards till the cleanup goes through.

[ndoo]

Rebased this branch on master as well as the demo branch of the next round of additions https://github.com/ndoo/meshtastic-firmware/compare/xiao-diy-cleanup...ndoo:meshtastic-firmware:xiao-nrf52840-e22

[ndoo]

Thank you, please keep in touch if there are any downstream impacts so I can have a look.