Add a common Ubuntu python module and make use of
it with the ubuntu.i386 script.
This is preparation for adding an Ubuntu script
ubuntu.aarch64.  Splitting out the common
logic such as build_image() will reduce duplication.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200601211421.1277-5-robert.foley@linaro.org>
Message-Id: <20200701135652.1366-9-alex.bennee@linaro.org>