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.

pkgs/misc/uboot/default.nix

@@ -29,6 +29,8 @@ stdenv.mkDerivation rec {
     sha256 = "0q2x1wh1f6rjh9rmcnkf28dxcvp9hkhi4vzspqkzamb6b3gp06ha";
   };
 
+  patches = [ ./vexpress-Use-config_distro_bootcmd.patch ];
+
   nativeBuildInputs = [ bc dtc ];
 
   configurePhase = ''