Auto-generate vendor ramdisk and boot image

They both still have a number of deficiences vs the make-built ones,
but this is a start.

Bug: 377563622
Test: Presubmits
Change-Id: If3329ae44633b00a8c2c2d15e3574087f74efe88
diff --git a/android/variable.go b/android/variable.go
index 88cf5a5..ed7e13b 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -612,6 +612,7 @@
 	// Boot image stuff
 	BuildingRamdiskImage            bool   `json:",omitempty"`
 	ProductBuildBootImage           bool   `json:",omitempty"`
+	ProductBuildVendorBootImage     string `json:",omitempty"`
 	ProductBuildInitBootImage       bool   `json:",omitempty"`
 	BoardUsesRecoveryAsBoot         bool   `json:",omitempty"`
 	BoardPrebuiltBootimage          string `json:",omitempty"`