use BOARD_PREBUILT_DTBOIMAGE in target-files-package

it's better to use the original $(BOARD_PREBUILT_DTBOIMAGE) instead
of AVB-padded $(INSTALLED_DTBOIMAGE_TARGET), the latter will be padded
to the size of /dtbo partition

Test: `make target-files-package` target file RECOVERY/recovery_dtbo is
    the original BOARD_PREBUILT_DTBOIMAGE
Test: `make otapackage` build pass and OTA file works fine

Change-Id: Iec8524d3a0544ddca89481dc613f076b36aeed25