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The hardware boot order on all platforms is: FSPI->EMMC->SD->USB. U-Boot SPL (our chainloader), however, is told by its DTB to boot in this order: SD->EMMC->FSPI. It then populates `RkAtagTypeBootDev` with the device it decided to boot from. We use this information to determine the drive we belong to, in order to write the variable store there. While this behavior is useful for testing, it should generally be avoided because it bypasses the SPL version we intended to ship with UEFI, which could lead to all sorts of issues. One such issue is that some SPL builds (namely Orange Pi's) flashed to SPI will happily boot from SD card while setting the ATAG to EMMC instead. Obviously, this leads UEFI to use the wrong device (or none at all if EMMC is missing) for writing variables. Since SPL only reads the variable store into memory from the actual boot device (SD card), settings will not persist. To address this, we'll no longer rely on that ATAG unless it indicates FSPI boot, in which case it would most likely be correct due to FSPI having priority in hardware - well, assuming there no further broken SPLs in the wild that might set it to FSPI while booting from SD :P. Instead, we'll look through the SD->EMMC devices (same order as SPL) to find a FIT image matching our own, indicating that's likely the boot device. Signed-off-by: Mario Bălănică <mariobalanica02@gmail.com>