rules: Fix by-path of mmc RPMB partitions and don't blkid them

Linux 3.10+ exposes RPMB (Replay Protected Memory Block) partitions of MMC
devices [1] ; trying to read them with blkid or other unspecific means will
cause kernel buffer I/O errors and timeouts. So don't run blkid on these.

Also ensure that /dev/disk/by-path creates proper symlinks and exposes the
-rpmb partition separately, instead of letting the "normal" partition symlink
point to the rpbm device (this is a race condition).

[1] http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=090d25fe224c0

https://launchpad.net/bugs/1333140