Clean up vendor image handling

Standardize symlinking /system/vendor -> /vendor for aosp_* devices,
since some /vendor binaries still use /system/vendor/... paths.

Support using a prebuilt vendor image and including it into all the
normal packaging steps.

Bug: 28987532
Change-Id: I27040e8a8d1df0777e16cd1e3c3a9f1b28695e96
diff --git a/core/envsetup.mk b/core/envsetup.mk
index 0a7472a..49ca154 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -173,6 +173,19 @@
 $(error TARGET_COPY_OUT_VENDOR must be either 'vendor' or 'system/vendor', seeing '$(TARGET_COPY_OUT_VENDOR)'.)
 endif
 PRODUCT_COPY_FILES := $(subst $(_vendor_path_placeholder),$(TARGET_COPY_OUT_VENDOR),$(PRODUCT_COPY_FILES))
+
+BOARD_USES_VENDORIMAGE :=
+ifdef BOARD_PREBUILT_VENDORIMAGE
+BOARD_USES_VENDORIMAGE := true
+endif
+ifdef BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE
+BOARD_USES_VENDORIMAGE := true
+endif
+ifeq ($(TARGET_COPY_OUT_VENDOR),vendor)
+BOARD_USES_VENDORIMAGE := true
+else ifdef BOARD_USES_VENDORIMAGE
+$(error TARGET_COPY_OUT_VENDOR must be set to 'vendor' to use a vendor image)
+endif
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