FAI for ARM image building

[Riku Voipio]

Hi,

I've been exploring fai-diskimage for building fastbootable
filesystems and disk images for 96boards form factor devices. Right
now we use live-build + linaro-media-create which becoming quite
outdated. FAI appears a good fit so far. Our current configs are at:
https://git.linaro.org/ci/fai.git/tree/

we can now make images but there are some bumps.

- fai-diskimage doesn't error out on failures. In our case, it is
imperative that the image build fails if packages are missing or
scripts fail.
- No apparent access from scripts or hooks to the output file
location. This would be useful for extracting artifacts (kernel etc)
from the disk image to the same directory where disk image is going.
- lack of tasks to hook into after the filesystems have been
unmounted. faiend task seems to run before it. The key use here is to
run img2simg or ext2simg to convert the rootfs image into fastbootable
android sparse image. We can make a wrapper script for it of course.
- The rather manual steps of copying files/. Having to DYI template
expansion and explicitly fcopy every single file is really annoying.
Right now users are tempted to just use herefile from the scripts
directly.

We haven't used the new cross-arch support in FAI 5.4 yet. I think it
has potential to be really useful for our endusers

Riku