With Python 3, YOTTA_BUILD_VCS_xxx constants are bytes objects not str

[dpgeorge]

When installing and using yotta with Python 3, the YOTTA_BUILD_VCS_ID and YOTTA_BUILD_VCS_DESCRIPTION macros automatically defined in yotta_build_info.h come out as bytes objects and hence are of the form b'...'. They should probably be just plain str objects so the "b" and quotes are not there.

Is yotta intended to be used with Python 3? To fix, I guess it's simply a matter of decoding to UTF-8 the output of git commands executed in lib/vcs.py.