Add ramdisk image.

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

Change-Id: I11389777c6bfb0c0d73bbb4c70091c1e70f44077
diff --git a/genrule/genrule.go b/genrule/genrule.go
index c5aaed2..a0008d3 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -550,6 +550,7 @@
 
 func (x noopImageInterface) ImageMutatorBegin(android.BaseModuleContext)                 {}
 func (x noopImageInterface) CoreVariantNeeded(android.BaseModuleContext) bool            { return false }
+func (x noopImageInterface) RamdiskVariantNeeded(android.BaseModuleContext) bool         { return false }
 func (x noopImageInterface) RecoveryVariantNeeded(android.BaseModuleContext) bool        { return false }
 func (x noopImageInterface) ExtraImageVariations(ctx android.BaseModuleContext) []string { return nil }
 func (x noopImageInterface) SetImageVariation(ctx android.BaseModuleContext, variation string, module android.Module) {