init: adding fallback paths for debug ramdisk

Currently the gki_4_19_pixel5 presubmit test uses an old
vendor_boot-debug.img from a release branch. Adding fallback
paths to load debug resources from /first_stage_ramdisk dir to
pass the presubmit.

This CL should be reverted later once the vendor_boot-debug.img
gets updated to store the debug resources on the root dir.

Bug: 186082603
Test: boot a device with boot-debug.img
Test: boot a device with vendor_boot-debug.img
Change-Id: I9fcd77fc5a60a15cff254e432e05f1c9122ad80d