Bundled themes: Add an alt attribute with the site title for header images linked to the home page.

props rianrietveld.
fixes #15926.
Built from https://develop.svn.wordpress.org/trunk@29842


git-svn-id: http://core.svn.wordpress.org/trunk@29606 1a063a9b-81f0-0310-95a4-ce76da25c4cd