Resolve "Implement air_syscall_shutdown_module()"

Closes #42

See merge request !9

e45b5178 Implements bsp_shutdown_core on the Zynq-7000

    Executes the necessary procedures to enter sleep mode as defined in the
    Zynq-7000 TRM
    Implements all auxiliary procedures to enter sleep mode
    Prints "AIR has been terminated by the application." message

a4603548 Implements air_syscall_shutdown_module

    It now calls bsp_shutdown_core