U-Boot loader and related files for the Radxa Rock Pi 4.

To install this bootloader on an sdcard just do:
dd if=/usr/local/share/u-boot/u-boot-rock-pi-4/idbloader.img of=/path/to/sdcarddevice seek=64 bs=512 conv=sync
dd if=/usr/local/share/u-boot/u-boot-rock-pi-4/u-boot.itb of=/path/to/sdcarddevice seek=16384 bs=512 conv=sync