In Lab7, we implement FAT32 file system with sd card
hardware device.

Implement the following features:
1. Implement FAT32 file system that meet the VFS interface.
2. Access sd card by simple sd card device driver (Provided by TA).
3. Mount the fat32 to VFS from sd card.
4. Read/Parse Master Boot Record(MBR) and FAT32 boot sector.
5. Open/Read/Write/Create a file in FAT32 and sd card.
6. Cache the directory entry read from device in memory for
   pathname lookup (Eletive 4-1).
7. Add some test files in "./sdcard" folder and build some
   test cases.

Initialize the memory management unit(MMU) and set up the
address spaces for the kernel and user processes
to achieve process isolation

1. Set up TCR_EL1 and mair_el1 system register for configuring MMU
2. Modify linker script to meet MMU configs
3. Set up identity paging.
4. Linear map kernel with three layer translation(secotion mapping, 2MB)
   granularity

Requirement2:
    - Implement user space paging.
    - Revisit fork() and exec() to map the same virtual address to
      different physical addresses for different processes.
    - Set ttbr0_el1 to switch the user space page table and
      address space in context switches.
    - Implement a simple page fault handler.
    - Write test cases for fork and exec syscalls.
Elective3, Imeplement Page Fault Handler & Demand Paging:
    - (Segmentation fault) If the fault address is not part of any
      region in the process’s address space, process will be killed.
    - (Demand paging) If it’s part of one region, map one page frame
      for the fault address. It's tested and occured by stack
      page allocation, because we not allocate page for stack when
      create process.

The reason why program run on hardware failure is that
uart read interrupt might be triggered continously when
we initalize rpi3 hardware. Sometimes there are a random char
is generated during booting rpi3 for no reason(We can't find the reason)