Copy custom bootimages verbatim

Regular boot images are generated by releasetools and added to
the target package. For custom boot images which depend on additional
parameters (such as DT - Flattened Device Tree), we copy the generated
boot image from $(PRODUCT_OUT) directly into the target package.

Original commit by bekriebel: https://github.com/CyanogenMod/android_build/commit/2169837037af499685e8b8290699ba476f608673

Change-Id: I5db80d4a0104f1e05c76d8a91540daf98bc825ac