Resolved! "How to create your own machine" in wiki and hits optee CFG_DRAM_SIZE bug
Following the instructions here: https://wiki.st.com/stm32mpu/index.php/How_to_create_your_own_machineAnd then making a trivial devicetree modification (enabling I2C5 output) fails to build on a fresh yocto install (via the "distribution package"). ...