MFC r331212,r331213: Move /boot/overlays to /boot/dtb/overlays

r331212: Move /boot/overlays to /boot/dtb/overlays

The former is fairly vague; these are FDT overlays to be applied to the
running system, so /boot/dtb is a sensible location to put it without
cluttering up /boot/dtb even further if desired.

r331213: Amend missed reference to /boot/overlays