Support for BLE setup

[ryanckulp]

the TRMNL PCB supports BLE, so we could make this an option for the WiFi credential handshake. requested by Noah in public comments.

our RPi prototype implemented a rough Web Bluetooth API option, but we replaced this with WiFi AP Captive portal.

[gdifiore]

Working on this

[gdifiore]

1. Is there a preference for another web-based UI?
   I have a prototype branch for app-based provisioning, it was easier to implement as a PoC. I do believe keeping a non-web-based option could be beneficial as well.
2. How should we determine which WiFi provisioning method to use?
   For example, should we have a dropdown in the device settings, or attempt BLE provisioning if the captive portal fails after three retries?

Also, for reference on increased binary size from added BLE libraries:

# Latest `main`
Checking size .pio\build\local\firmware.elf
...
RAM:   [===       ]  28.2% (used 92404 bytes from 327680 bytes)
Flash: [=======   ]  65.5% (used 1288164 bytes from 1966080 bytes)
# WIP BLE WiFi Provisioning (nimBLE library)
Checking size .pio\build\local\firmware.elf
...
RAM:   [===       ]  30.0% (used 98356 bytes from 327680 bytes)
Flash: [=======   ]  74.1% (used 1457082 bytes from 1966080 bytes)

[Omeryl]

Oh yikes. 10% of the flash spent just for provisioning is kind of scary to me since so much flash is already spent on the provisioning for captive portal.

[gdifiore]

Agreed, that's why I paused after getting it working. If someone knows a smaller library than nimBLE I can take a look, but it may be the smallest possible.

[gdifiore]

@ryanckulp Thoughts/ideas on this?

[stevekennedyuk]

simpleBLE? Or direct use of the ESP-IDF and remove the unnecessary bits

[gdifiore]

simpleBLE doesn't appear to support creating Bluetooth characteristics, which is required for receiving data from from the input (either app or web page).

The minimum implementation with ESP-IDF brought the FW binary size up to around 94%(!) of flash. The linker should already "remove unnecessary bits" from the lib, so manually editing code or compiler flags wouldn't change the final binary size.

[0PandaDEV]

Am I right that this would make it possible to use the trmnl completely without Wi-Fi all over BLE if it were to be implemented? Because that would be a game changer for me as I turn my Wi-Fi off overnight but still need the data.