Support to set up TARGET_COPY_OUT_VENDOR in board config.

We first define TARGET_COPY_OUT_VENDOR as a placeholder. In product
config makefiiles we actually get the placeholders in
PRODUCT_COPY_FILES. A device can set up TARGET_COPY_OUT_VENDOR in its
BoardConfig.mk. We substitute the placeholder with the real
TARGET_COPY_OUT_VENDOR value after loading the BoardConfig.mk.
With this change, we can support building vendor stuff to
system.img (the default) or a separate vendor.img.

Bug: 16515152
Change-Id: I5b601d7a8b34fe032a1bac02aa5c204a3765691d