platforms: Build flat kernel image for RISC-V

Newer bootloaders for RISC-V (i.e., OpenSBI + U-Boot) support
flat and compressed kernel images but not vmlinux. Therefore,
let's build "Image" like what we do with aarch64.

Also copy DTBs while we are at it.

lib/systems/platforms.nix

@@ -481,9 +481,10 @@ rec {
   riscv-multiplatform = {
     linux-kernel = {
       name = "riscv-multiplatform";
-      target = "vmlinux";
+      target = "Image";
       autoModules = true;
       baseConfig = "defconfig";
+      DTB = true;
       extraConfig = ''
         FTRACE n
         SERIAL_OF_PLATFORM y