/*linux板级设备的初始化过程
start_kernel() -> setup_arch(&command_line) -> mdesc = setup_machine(machine_arch_type) -> list = lookup_machine_type(nr) -> return struct machine_desc. start_kernel() -> setup_arc
start_kernel() -> setup_arch(&command_line) -> mdesc = setup_machine(machine_arch_type) -> list = lookup_machine_type(nr) -> return struct machine_desc. start_kernel() -> setup_arc