Fails with OPENTHREAD = off. Error message:
LINK build/FIPY/release/application.elf
build/FIPY/release/application.a(modlte.o):(.literal.TASK_LTE_UPGRADE+0xc): undefined reference to `xMeshTaskHndl'
xMeshTaskHndl is not defined in the sources and not in esp-idf, branch idf_v3.1. Updating esp-idf does not change the picture.
Building for LoPy4, LOPY, GPY and WiPy works.