Specify dirs property in generated ramdisk filesystem module

To match with that of the make generated ramdisk partition image.

Test: unpackage ramdisk.img and inspect directories
Bug: 379190521
Change-Id: If4b1b448e1559077198809dce29e3c3a89f9e653
diff --git a/android/variable.go b/android/variable.go
index ed7e13b..2f72e1a 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -621,6 +621,7 @@
 	BoardInitBootimagePartitionSize string `json:",omitempty"`
 	BoardBootHeaderVersion          string `json:",omitempty"`
 	TargetKernelPath                string `json:",omitempty"`
+	BoardUsesGenericKernelImage     bool   `json:",omitempty"`
 
 	// Avb (android verified boot) stuff
 	BoardAvbEnable          bool                                `json:",omitempty"`