Set the bootcmd in u-boot

In QEMU the kernel is loaded at the ${kernel_addr_r} address and we need
to use booti instead of bootm.

overlay.nix

@@ -22,8 +22,9 @@ final: prev:
   qemu = prev.qemu.override { rutabagaSupport = false; };
 
   uboot-custom = prev.ubootQemuRiscv64Smode.override {
+    # Override preboot to set 'bootcmd' directly to the kernel address in RAM
     extraConfig = ''
-      CONFIG_CMD_QFW=y
+      CONFIG_PREBOOT="setenv fdt_addr ''${fdtcontroladdr}; fdt addr ''${fdtcontroladdr}; setenv bootcmd booti ''${kernel_addr_r} - ''${fdtcontroladdr};"
     '';
   };