Add a workflow to build and import the official iPXE images for AWS
EC2. Treat this as a workflow that must be triggered manually, since
importing is prone to failure for reasons unrelated to the state of
the codebase (e.g. the creation of new regions, or an explosion at a
data centre) and so should not result in CI failures being reported
against specific commits.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
