mkosi: Build minimal images and enable related integration tests

mkosi: Build minimal images and enable related integration tests

This commit adds definitions to build the minimal_0 and minimal_1
images with mkosi and includes them into the system image. We also
move the building of the various app-xxx and similar images that are
extremely minimal into the tests itself by moving the related logic
from install_verity_minimal() into a new function
install_extension_images() in util.sh. Because the mkosi /usr is
read-only, we now place the extension images in /tmp instead of
/usr/share.

Co-authored-by: Richard Maw <richard.maw@codethink.co.uk>
Co-authored-by: sam-leonard-ct <sam.leonard@codethink.co.uk>