fix(build_image): Only clear UUID on rootfs/usr filesystems.

The funky UUID and other special settings should only be applied to
coreos-rootfs and coreos-usr partitions which will never be fscked. When
STATE becomes ROOT in -usr images it gets fsked while mounted read-only
and fsck updates the filesystem's UUID if it is blank. Turns out this
causes disagreement between the kernel and the disk leading to bad
things. A related issue was fixed in a newer version of tune2fs but
unless I missed it the same bugfix didn't make it into e2fsck so
updating wouldn't resolve the issue.

http://e2fsprogs.sourceforge.net/e2fsprogs-release.html#1.42.9

This should end the disk corruption issues we've been having with -usr