fetch2: fix pickle issues while switching from master to krogoth

fetch2: fix pickle issues while switching from master to krogoth

While switching from master to krogoth build with a common download directory,
got a large number of warnings like the one listed below:

WARNING: freetype-2.6.3-r0 do_fetch: Couldn't load checksums from
donestamp /home/maxin/downloads/freetype-2.6.3.tar.bz2.done: ValueError
(msg: unsupported pickle protocol: 4)

These warnings are caused by the difference in pickle module
implementation in python3(master) and python2(krogoth). Python2 supports
3 different protocols (0, 1, 2) and pickle.HIGHEST_PROTOCOL is 2 where as
Python3 supports 5 different protocols (0, 1, 2, 3, 4) and
pickle.HIGHEST_PROTOCOL is obviously 4.

My suggestion is to use 2 since it is backward compatible with python2
(all the supported distros for krogoth provides python2 which supports
pickle protocol version 2)

Signed-off-by: Maxin B. John <maxin.john@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>