File System: Dependency Injection and Mock

1. Add an abstract FileSystemInterface for Dependency Injection
2. Add FileSystemWrapper for real file system operations, which is
called in produciton

File System: Dependency Injection and Mock

1. Add an abstract FileSystemInterface for Dependency Injection
2. Add FileSystemWrapper for real file system operations, which is
called in produciton
3. Add FileSystemMock for unit test mock only
4. Add bm_mode_transition_unittest.cpp to cover file system based bm
mode transition

Tested:
1/14 eth OK 0.22s
2/14 flash OK 0.21s
3/14 machine OK 0.12s
4/14 bmc_mode OK 0.05s
5/14 cable OK 0.27s
6/14 cpld OK 0.25s
7/14 entity OK 0.24s
8/14 google_accel_oob OK 0.20s
9/14 pcie OK 0.11s
10/14 poweroff OK 0.09s
11/14 psu OK 0.08s
12/14 pcie_bifurcation OK 0.07s
13/14 bm_mode_transition OK 0.29s
14/14 handler OK 0.44s

Ok: 14
Expected Fail: 0
Fail: 0
Unexpected Pass: 0
Skipped: 0
Timeout: 0

Change-Id: I8d0d9096fbeee7af1946d7e73f548e4a249d3bee
Signed-off-by: Hao Zhou <haoooamazing@google.com>

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