mkzfs.py: Use less memory during image build

Fixes #198.

Signed-off-by: Sanidhya Kashyap <sanidhya.k@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
upload_manifest.py: Use less memory during image build

Fixes #198.

Signed-off-by: Sanidhya Kashyap <sanidhya.k@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>