Mount basic file systems during customize

Created by: SunilMohanAdapa

This allows plinth --setup to run properly.

This patch was actually tested on top of my cubietruck-fixes. But it is really independent. So, I am submitting it separately. To test, I have built and amd64 image with corresponding patch to run plinth --setup in freedombox-setup. The setup log showed that mounts happed properly and so did unmounts. Making use of the mounts, Plinth setup also ran as expected.