novena-image: Add -q to avoid reformatting a partition

When an apt-get fails, we don't need to reformat/debootstrap the whole
thing.  Therefore, we can do a --quick install to skip these steps.

Signed-off-by: Sean Cross <xobs@kosagi.com>
novena-image: Add mapper support

If a file is specified that is not a block device, switch to mounting
it as a loopback device.

Signed-off-by: Sean Cross <xobs@kosagi.com>