Try to workaround issue when creating Mac dmg image

Sometimes one would get an error "hdiutil: resize request is above
maximum size allowed." which might have to do with autostretch limits.
Passing the -stretch argument with a sufficiently large size should
make resizing succeed.

Change-Id: I3103ce38e809c2429bed6847ead03f28bfa1f54a
Reviewed-by: Eike Ziller <eike.ziller@digia.com>