einj.h: add a header file to declare common EINJ related operations

Lots of files declare the same EINJ related macros like EINJ_ETYPE and
functions like wfile(), include the same header files.

To simplify the code and make it easier to maintain, move all common EINJ
related operations to a header file.

[Tony: Move the code out of einj.h and into new file einj.c]

Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>