Do not use stdout to pass data from python to the linaro-media-create shell script

[Guilherme Salgado]

In l-m-c, setup_partitions() evals all the output of a python script, but that script calls out to several external utilities which may write to stdout and depending on what they write, eval will fail. One alternative to avoid that is to have the python script write to a file just the bits we want to pass back to shell.