Fix CreatePatch.py to use ss_bsdiff in the common/bin

Fix CreatePatch.py to use ss_bsdiff in the common/bin

In the python script CreatePatch.py, ss_bsdiff is checked whether it is
exist in the /usr/local/bin/ss_bsdiff or not.
If not exist, then it checks if it exists under "../../common/bin".
Script makes new path using statement below:
 > os.path.join("../../common/bin", "/usr/local/bin/ss_bsdiff")
But the return value of the function above is not
 > "../../common/bin/usr/local/bin/ss_bsdiff"
but
 > "/usr/local/bin/ss_bsdiff"
so the script is just checking about "/usr/local/bin/ss_bsdiff" twice.

To fix it, a variable DIFF_UTIL_NAME is added to store the name of the
diff util and check if it exist under "../../common/bin" if
/usr/local/bin/ss_bsdiff is not exist.

Change-Id: I138b54b7dc77a47130435e9034f594b99afa35aa
Signed-off-by: SangYoun Kwak <sy.kwak@samsung.com>