U-Boot: Patch Versatile Express to use config_distro_bootcmd.h

With this patch in place, ARMv7 NixOS can be booted in QEMU:

qemu-system-arm -kernel u-boot -M vexpress-a9 -serial stdio -sd nixos-sd-image-armv7l-linux.img -m 1024

...with all the features that boot.loader.generic-extlinux-compatible
supports, like the boot generation menu and seamless kernel upgrades
in the VM.