Add INSTALLED_DTBOIMAGE_TARGET to droidcore-unbundled

dtbo.img omitted when BOARD_AVB_ENABLE is disabled.
To copy dtbo.img to PRODUCT_OUT, INSTALLED_DTBOIMAGE_TARGET added
to droidcore-unbundled.

Test: Build w/ BOARD_AVB_ENABLE=true and false
Bug: 379030670
Signed-off-by: Junki Min <joonki.min@samsung.com>
Change-Id: Ie59d572ec5e5216cfc3b7ca7ff735a3ac0caab13