fastboot: Update the help message with -b argument

            "  -p <product>                             specify product name\n"

            "  -c <cmdline>                             override kernel commandline\n"

            "  -i <vendor id>                           specify a custom USB vendor id\n"

            "  -b <base_addr>                           specify a custom kernel base address\n"

        );

    exit(1);

}