arch-riscv: correctly pass arguments to kernel with new bootloader+kernel (#635)

The [PR](https://github.com/gem5/gem5/pull/390) adds support for new
bootloader and linux kernel. However after applying the changes the
arguments are not passed correctly to the kernel resulting in kernel
panic during simulations. This commit fixes the issue.

src/python/gem5/components/boards/riscv_board.py

@@ -256,7 +256,7 @@ class RiscvBoard(AbstractSystemBoard, KernelDiskWorkload):
             root.append(node)
 
         node = FdtNode(f"chosen")
-        bootargs = " ".join(self.get_default_kernel_args())
+        bootargs = self.workload.command_line
         node.append(FdtPropertyStrings("bootargs", [bootargs]))
         node.append(FdtPropertyStrings("stdout-path", ["/uart@10000000"]))
         root.append(node)