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.
868e287e71