设备驱动模型：完善platform bus相关内容。并注册串口到sysfs (#403)

* 完成初始化platform bus
* 删除旧的sysfs
* 把uart驱动移动到tty/serial文件夹下
* 完成将串口挂载到sysfs
* 修复vfs系统调用未能follow symlink的问题
* 修复shell未能正确获取pwd的问题

kernel/src/arch/x86_64/mod.rs

@@ -20,3 +20,5 @@ pub use self::pci::pci::X86_64PciArch as PciArch;
 pub use self::mm::X86_64MMArch as MMArch;
 
 pub use interrupt::X86_64InterruptArch as CurrentIrqArch;
+
+pub use crate::arch::asm::pio::X86_64PortIOArch as CurrentPortIOArch;