x86: zImage: add Intel MID platforms support

Intel MID platform boards have special treatment, such as boot parameter
setting.

Assign hardware_subarch accordingly if CONFIG_INTEL_MID is set.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Vincent Tinelli <vincent.tinelli@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>