python2 create_manifest.py: fix trailing whitespace in json

python2 create_manifest.py: fix trailing whitespace in json

The json.dumps function adds trailing whitespace when using
indent, because the default separator is not ','.

The workaround [1] is to set the separators to be ',' and ': ',
e.g. separators=(',', ': ')

[1] https://hg.python.org/cpython/rev/78bad589f205

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>