Add the missing dependency on BOARD_PREBUILT_DTBOIMAGE.

Test: Use a target that defines `BOARD_INCLUDE_RECOVERY_DTBO := true`
      and BOARD_PREBUILT_DTBOIMAGE. Touching a prebuilt dtbo file
      triggers rebuilding of recovery image.
Change-Id: Icf44032fb29542c6836a401c79fb85345ad6cc3c