nRF52 has its own upload_program that runs before the default dispatch
in esphome.__main__. The mcumgr/BLE OTA path reads the MCUboot-signed
update image from CORE.relative_pioenvs_path(name, 'zephyr',
'app_update.bin'), which assumes a local Zephyr build tree.
Consult CORE.prebuilt_artifact_path('app_update.bin') first so the
dashboard's transparent BLE install on a Bluetooth proxy can flash a
prebuilt update image without compiling locally.
Serial uploads on non-MCUboot bootloaders still go through
adafruit-nrfutil via _upload_using_platformio and are out of scope here;
they need their own bypass to work with --prebuilt-dir (tracked
separately).
Issue: esphome/device-builder#572
J. Nick Koston