Add ramdisk image.

It is similar to recovery image.
Test: m nothing -j

Change-Id: I11389777c6bfb0c0d73bbb4c70091c1e70f44077

rust/rust.go

@@ -85,6 +85,10 @@ func (mod *Module) CoreVariantNeeded(ctx android.BaseModuleContext) bool {
 	return true
 }
 
+func (mod *Module) RamdiskVariantNeeded(android.BaseModuleContext) bool {
+	return mod.InRamdisk()
+}
+
 func (mod *Module) RecoveryVariantNeeded(android.BaseModuleContext) bool {
 	return mod.InRecovery()
 }
@@ -152,6 +156,10 @@ func (mod *Module) Toc() android.OptionalPath {
 	panic(fmt.Errorf("Toc() called on non-library module: %q", mod.BaseModuleName()))
 }
 
+func (mod *Module) OnlyInRamdisk() bool {
+	return false
+}
+
 func (mod *Module) OnlyInRecovery() bool {
 	return false
 }