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/logical_partition.go b/filesystem/logical_partition.go
index 16b6037..20d9622 100644
--- a/filesystem/logical_partition.go
+++ b/filesystem/logical_partition.go
@@ -209,6 +209,10 @@
 	return l.output
 }
 
+func (l *logicalPartition) SignedOutputPath() android.Path {
+	return nil // logical partition is not signed by itself
+}
+
 var _ android.OutputFileProducer = (*logicalPartition)(nil)
 
 // Implements android.OutputFileProducer