Clean up /var in the filesystem tree

Some elements install files into /var during the BuildStream build.

This causes warnings when deploying the tree via ostree admin deploy. The warnings are harmless, as long as the files in /var can be dropped without breaking stuff.

There are a few ways to solve this, with different tradeoffs:

• rm -rf /var/* when preparing the final tree
• Find each component that puts files in /var, remove those specific files at component install time
• As above, but configure the component to not install those files in the first place, which may require some patching or upstream fixes

[dbnicholson]

In eos-ostree-builder, eventually we just did rm -rf /var; mkdir /var. Whatever you leave in there is definitely not being used at runtime since it just gets mounted over. We did have a couple other tools that expected to find some things in the commit's /var (in particular, our container builder), but they've been fixed by now.

The only other thing I was worried about was the "documentation" of things that packages expected to have in /var that we'd need to add some runtime handling for (tmpfiles snippet, boot service, etc). I decided adding rm -rvf would be good enough for someone who wanted to go looking.