pre-populate /srv/tftpboot repos in overlay filesystem

The /srv/tftpboot/repos in the overlay are now pre-populated by
bind-mounting from the image-building host.
add build-custom to run at end of appliance creation
the RAM disk needs at least 5GB

... and that's without the software repositories embedded.
mention packer
fill out image-building instructions (WIP)
tmpfs RAM disk needs a LOT when all the repos are overlaid
allow passing parameters to kiwi (e.g. --type)
switch to .vmdk for virtualbox
fix to save .vmdk files