init: Look for super partition only on a boot device

Init code is bailing out as soon as `super` partition was
found in the system, ignoring rest of uevents.

In case given device contains multiple boot sources, `super`
partition as well as all other partitions shall be taken
from the same boot source, instead of relying on
`which uevent came first`.

Bug: 309244873, 349144493
Test: Plug secondary USB boot device to device that supports multiple
      boot sources. Select boot from USB. Device boots properly.

Signed-off-by: Jan Dabros <dabros@google.com>
Signed-off-by: Konrad Adamczyk <konrada@google.com>

Change-Id: I70eb7d4223258ec273faa523cb67ddab0b7c32a0