A number of passt/pasta testcases have initial steps which are just about
building images or other assets we need for the test proper. Repeating
these for each test run can be quite costly.
This patch makes a start on moving this sort of test asset building to
a separate phase before running the tests proper. For now just add a
Makefile to handle the asset building (although it doesn't build
anything yet), and make the path where we'll be building the assets
available to the tests.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
