init: Remove special-case hacks for the super partition.

Rather than special-casing super in the uevent listener, call Realpath()
after we've generated its symlink. This allows us to generalize the
uevent listener code.

Also, rename "metadata partition" to "super partition" in a few places,
as this terminology is a bit dated.

Bug: 150315914
Test: first-stage init boots
Change-Id: I2bf3bebf3360f097582df7fba95f3c1753feb2e2