Revert^3 "init: Look for super partition only on a boot device"

This reverts commit 5bfb9367.

Reason for revert: b/376468452 and trusty boot up on arm64. This CL is causing a lot of troubles (now only on emulator, but may affect more devices in field) and shall be reverted. Desktop team will handle support for selecting single boot source (while having more than one) as part of boot_part_uuid support (at aosp/3318438).  

Change-Id: I2804c119631f592d0862f3472ffe18dbb23b17e5