Don't check the md5sum of linux-chromebook source tarballs

Make the linux-chromebook package feasible to build again.

Commit af0a9081f743265a71db625633bb2e58481a88e6 introduced an md5sum for the chromebook archive .tar.gz tarball pulled from chromium.googlesource.com, but even though these tarballs are specific to a commit they are regenerated regularly (on each download?) with new timestamps, so the md5sum changes.

I just put back in the SKIP value that had been present in the previous commit 303fd6d7f570d635913fd2133703c2cb3ae19007 (since f6d5381f0abf6e629ab1f39173fb2dbf6598a5a4), and clarified the comment to avoid this disappearing in the future.