Move TARGET_COPY_OUT_VENDOR to BoardConfig.mk

Now the build system supports TARGET_COPY_OUT_VENDOR in BoardConfig.mk.
This removes the burden that we need to set up TARGET_COPY_OUT_VENDOR in
the leaf product config makefile before including any other product
makefile.

Bug: 16515152
Change-Id: I876cd6c0e1fd20e980bcdd15b6ef0a91fd47fc78
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 03353da..9c7ba10 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -16,6 +16,8 @@
 
 # Use the non-open-source parts, if they're present
 -include vendor/htc/flounder/BoardConfigVendor.mk
+# Build a separate vendor.img
+TARGET_COPY_OUT_VENDOR := vendor
 
 TARGET_ARCH := arm64
 TARGET_ARCH_VARIANT := armv8-a