This patch is based on contribution by Krunal Suresh Bauskar from Huawei.
Add include/my_checksum.h and move ha_checksum and my_checksum() here.
Include my_checksum.h where needed. Remove checksum.cc which is no longer needed.

Change-Id: I7c25956adae4c556e7cd98580a9f836f33dac3eb