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Pinning the U-Boot build environment for the IOT-DIN-IMX8PLUS

imx-boot is not one repository — it is U-Boot, ARM Trusted Firmware, NXP's imx-mkimage and the LPDDR4 training blobs, assembled together. Get one of them at the wrong version and it builds cleanly and fails to train memory. How I pinned all four using the board's own boot log.