There are a couple of bugs already reported on this - https://bugs.launchpad.net/ubuntu/+source/yaboot/+bug/1400030 and https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=784387
Neither really provide a step by step guide to overcoming this, although we can now tell that your device line is wrong in your yaboot.conf. I would also suggest you need to add an ofboot line too.
It’s a bit tricky because your powermac doesn’t use the ‘normal’ hd alias for its hard drive. I’ve no idea what powermacs use, but from the debian bug report sd1 is a possibility. So from openfirmware the command to load yaboot would be
boot sd1:2,yaboot
However, this still isn’t going to boot the system because of the broken device line in your yaboot.conf.
What I would suggest you do is this:
From a live 16.04 DVD mount your installed root partition For example,
sudo mount /dev/sda3 /mnt
Then edit your yaboot.conf:
sudo nano /mnt/etc/yaboot.conf
Delete the device line completely and replace it with the following two lines
ofboot=&device;:&partition;
nonvram
Then run the ybin command, but specify the yaboot.conf to use:
sudo ybin --config /mnt/etc/yaboot.conf -v
It’s a bit of a bodge, but this should get the system to boot provided your root partition is on the same hard drive as your bootstrap partition.
If you can figure out the alias or full ofpath to your hard drive then you can edit the device= and ofboot= lines appropriately and get rid of the nonvram line. I would recommend you try to do this. e.g.
device=sd1:
ofboot=sd1:2