Workaround NBD disconnection problem on shutdown/reboot

Calling nbd-client -d on root device during shutdown sequence worked
fine with 3.2 kernel but it seems to be much less ok for 3.17 kernel.

Tried several things including pivoting back to an initramfs but things
still do not work in a satisfactory fashion. I will continue testing but
need a good enough workaround for now so people using the image will
stop hitting the same bug over and over for no good reason.
Add README found in image

Details modifications done on top of armv7 stage3.
Add files to create a clean image