WIP: Change to use script element / prepare image approach

Currently blocked by freedesktop-sdk/freedesktop-sdk#990 (closed)

Hitting collect_initial_scripts problem at https://gitlab.com/celduin/bsps/example-app/-/jobs/558784238

Using the prepare image script provided by FDSDK, various config can be applied during the deployment of the final image. This also removes the need to depend on the genimage plugin.

The first commit was a near identical copy of example overlay app raspi change celduin/crash/bsp-playground/example-overlay-app!7 (merged)

The second commit was a move towards using Qemu+9p This commit managed to use root filesystem vm/minimal/virt.bst from fdsdk on bst 1.4 but fails on bst2 as mentioned above.