kernel: Include DTBOs from OEM/ODM folder in base DTB

It can be the case where OEM/ODM decided to put the main DTBOs
outside vendor/qcom (e.g. vendor/oplus). Support such usecase
by generically allowing all .dtbo files found in all first-level
subdirs to be moved to base DTB folder.

Change-Id: I154d401b560535d1fa8cd7ed4965dd34a4cf75f2