[Feature request] Enable nRF52840 firmware update from within Android app

[tropho23]

Respectfully requesting the ability to flash nRF52840 firmware from within the app. This is already possible via manual process, and is relatively easy to do using Android and a USB cable. Having a guided process to help 'normal' users do this would be wonderful.

Such a guided process could be as follows:

1. Under Radio Configuration, near the bottom of the list add a button labeled 'Update nRF52840 firmware' (or 'Update device firmware' with a warning popup that it only works for nRF52840 devices and not ESP32 devices)
2. Once pressed, the app asks user to specify the firmware file that should be already downloaded to the device; include a hyperlink to the Meshtastic firmware download site, perhaps
3. The app either accepts the correct .uf2 firmware file or extracts it from the larger firmware .zip file (extracting behind the scenes would be better for average users)
4. The app prompts the user to connect the nRF52840 device to the mobile device via USB cable
5. The app prompts the user to press the nRF52840 device's reset button twice to enabled DFU/bootloader mode
6. The app copies the specified .uf2 firmware file to the 'RAK4631' removable drive
7. The app informs the user the nRF52840 device has been successfully flashed, and prompts the user to disconnect the nRF52840 device from the mobile device
8. The app asks the user if they want the app to clean up downloaded firmware files (delete the .uf2 and/or larger firmware .zip file; requires file modify permissions)