Using the instructions from the L4T R24.2 documentations I have successfully built a custom u-boot. I did this because I needed to add/enable the RTL8152 driver during U-boot for PXE booting.
I copied over 3 files; u-boot.bin, u-boot.dtb, u-boot-dtb.bin to Jetson_For_Tegra/bootloader/t210ref/p2371-2180 (which is my board name directory).
I then ran sudo ./flash.sh -k EBT p2371-2180 mmcblk0p1 to flash only the u-boot in the emmc for the TX1. After flashing my custom U-boot onto my TX1 nothing changed, its as if I didn’t flash my TX1 at all.
Does ./flash.sh overwrite my current u-boot with the custom u-boot that i made?