stdout and stderr will default to subprocess.PIPE and subprocess.STDOUT
respectively (which is the expected behavior from most of the existing
callers), unless caller specifies any of them.
Test: `m dist`
Test: python -m unittest \
test_common \
test_add_img_to_target_files \
test_ota_from_target_files \
test_validate_target_files
Change-Id: I43b3f08edfa8a9bcfe54baf9848dc705c048e327