Specify dtb_prebuilt property in the autogen vendor_boot.img

prebuilt_etc_modules_gen currently elides entries that are not installed
within partition directories but are installed in the product root.
However, dtb.img is copied from source via PRODUCT_COPY_FILES and
installed in the product root and then used for vendor_boot.img
generation.

This change creates the filegroup module from the PRODUCT_COPY_FILES for
the source dtb.img to utilize it in the vendor_boot.img generation, but
not actually install the file. We may consider installing this file if
this is essential in runtime.

Test: inspect the ninja command of the vendor_boot.img
Bug: 379945468
Change-Id: I760b8eb9c15b74b124828ea041ca106f8355efd4
diff --git a/android/variable.go b/android/variable.go
index e666530..6eb8a22 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -627,6 +627,7 @@
 	BoardUsesGenericKernelImage     bool   `json:",omitempty"`
 	BootSecurityPatch               string `json:",omitempty"`
 	InitBootSecurityPatch           string `json:",omitempty"`
+	BoardIncludeDtbInBootimg        bool   `json:",omitempty"`
 
 	// Avb (android verified boot) stuff
 	BoardAvbEnable          bool                                `json:",omitempty"`