Skip building boot-(test-harness|debug).img if not needed

boot-(test-harness|debug).img is not needed for devices with
a /vendor_boot partition. They can use vendor_boot-(test-harness|debug).img
instead.

(Cherry-picked with Ic032b8594f776f911d7b6345a97d64fed930d890 to
solve merge conflict.)

Bug: 200945738
Bug: 196001476
Test: make then checks $OUT/*.img
Change-Id: If76df0329c96f718ebf0a0ac138cde3ae859a6c6
Merged-In: If76df0329c96f718ebf0a0ac138cde3ae859a6c6
(cherry picked from commit 4bb4d1fb)