Allow system images larger than 2GiB.

Python 2.7's zipfile implementation wrongly thinks that zip64 is
required for files larger than 2GiB. We can work around this by
adjusting their limit. Note that `zipfile.writestr()` will not work
for strings larger than 2GiB. The Python interpreter sometimes rejects
strings that large (though it isn't clear to me exactly what
circumstances cause this). `zipfile.write()` must be used directly to
work around this.

This mess can be avoided if we port to python3.

Bug: 18015246
Change-Id: I8a476d99c5efdef6ea408373b706e9fbd3a798be