stdlib: Give user's disk_device priority when setting root val (#1467)

In `get_default_kernel_root_val()`, now prioiritizes the explicit
disk_device passed from the user over the default implemented by the
board.

Also adjusts syntax for selecting this value in
`set_kernel_disk_workload()` for consistency.

It seems that the common use case for setting `disk_device` is that
there is a mismatch between where the disk image is mounted and where
the board expects it by default. In this case, it also seems common that
the root partition will be on this explicit device as well.

In cases where this is not true, explicit kernel arguments can be used
to define the distinct disk device apart from the root. However, this
seems less common than the above so, in that case, it would be easier to
tie these together.

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

@@ -126,16 +126,15 @@ class KernelDiskWorkload:
     ) -> str:
         """
         Get the default kernel root value to be passed to the kernel. This is
-        determined by the value implemented in the ``get_disk_device()``
-        function, and the disk image partition, obtained from
-        ``get_disk_root_partition()``
+        determined by the user-passed or board-default ``disk_device``, and
+        the disk image partition obtained from ``get_disk_root_partition()``.
 
         :param disk_image: The disk image to be added to the system.
 
         :returns: The default value for the ``root`` argument to be passed to the
                   kernel.
         """
-        return self.get_disk_device() + (
+        return (self._disk_device or self.get_disk_device()) + (
             self.get_disk_root_partition(disk_image) or ""
         )
 
@@ -196,11 +195,7 @@ class KernelDiskWorkload:
             " ".join(kernel_args or self.get_default_kernel_args())
         ).format(
             root_value=self.get_default_kernel_root_val(disk_image=disk_image),
-            disk_device=(
-                self._disk_device
-                if self._disk_device
-                else self.get_disk_device()
-            ),
+            disk_device=self._disk_device or self.get_disk_device(),
         )
 
         # Setting the bootloader information for ARM board. The current