novena-image: Check that packages exist before starting

Pre-check that packages specified by -a or --add-deb exist before
we attempt to install them.  This is just a bit of sanity-checking
before we do the bigger stuff.

Signed-off-by: Sean Cross <xobs@kosagi.com>
novena-image: Generate fstab, depending on media

For MMC media, point fstab at /dev/disks/by-path/, and for sata assume
it's at /dev/sda.

Signed-off-by: Sean Cross <xobs@kosagi.com>
examples: Update example package list again

Still tweaking it, but at least now it doesn't include all of libreoffice!

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