Add vbmeta module type

The module type is to create vbmeta image out of other partitions.

Bug: 180676957
Test: m microdroid_vbmeta microdroid_vbmeta_system
Inspect the built image using `avbtool info_image --image <image>`

Change-Id: Iac92e9ab1640dcd488af69842e09850a91262bf1
diff --git a/filesystem/bootimg.go b/filesystem/bootimg.go
index ce1c855..eb47e10 100644
--- a/filesystem/bootimg.go
+++ b/filesystem/bootimg.go
@@ -263,6 +263,13 @@
 	return b.output
 }
 
+func (b *bootimg) SignedOutputPath() android.Path {
+	if proptools.Bool(b.properties.Use_avb) {
+		return b.OutputPath()
+	}
+	return nil
+}
+
 var _ android.OutputFileProducer = (*bootimg)(nil)
 
 // Implements android.OutputFileProducer