post-createfs.sh: don't ignore errors

When fwtool fails, don't ignore the error and continue.
Remove rootfs.tar.gz creation to save time/space

For images with a lot of packages enabled, the tar.gz creation actually
takes a while. It isn't necessary since we can use the .tar file that's
created automatically anyway. This commit cleans that up.
Update unionfs download site
Refactor demo image creation to be same as SDK usage

Previously, the demo image was created using buildroot's tools and this
made it slightly different than what was created with normal projects.
This patch fixes this. The main change is that the output ext4 file
system will have similar parameters. This makes the demo image slightly
smaller.
README.md: add dependency on libncurses

This seems to be needed for host-erlang
Run resize2fs to reduce the rootfs partition significantly

This change removes the slack that had been needed to make up for not
being able to pre-calculate the number of blocks required for an image.
For the minimal image, it reduces the size from 16 MB to a little over
13 MB, so it will probably be felt in SDCard programming time. I think
that bigger images will usually be more significantly impacted.