[Skiboot] lcove coverage for unittests

[Stewart Smith]

I've created a gh-pages branch up on github, which is automagically
served up at: http://open-power.github.io/skiboot

Specifically, I've run "make coverage-report" and published it here:

http://open-power.github.io/skiboot/coverage-report/

I plan to (at some point) automagically keep it up to date.

Basically, you can see there's some areas we're missing test coverage
for. Bonus points to those who provide patches :)