init: Move the stripping of "/devices" and "/devices/platform/" to a helper

A future change will want the same stripping when looking for USB boot
devices. Move the stripping down to the helper.

This change is intended to be a no-op and just a reorganization.

Bug: 316324155
Test: See boot devices still found
Change-Id: I025d9d68fedf652055454cbd93e15f480b6056dd