Add ramdisk image.

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

Change-Id: I11389777c6bfb0c0d73bbb4c70091c1e70f44077
diff --git a/cc/genrule.go b/cc/genrule.go
index 548d5f2..155e410 100644
--- a/cc/genrule.go
+++ b/cc/genrule.go
@@ -25,6 +25,7 @@
 
 type GenruleExtraProperties struct {
 	Vendor_available   *bool
+	Ramdisk_available  *bool
 	Recovery_available *bool
 }
 
@@ -62,6 +63,10 @@
 	return Bool(g.Vendor_available) || !(ctx.SocSpecific() || ctx.DeviceSpecific())
 }
 
+func (g *GenruleExtraProperties) RamdiskVariantNeeded(ctx android.BaseModuleContext) bool {
+	return Bool(g.Ramdisk_available)
+}
+
 func (g *GenruleExtraProperties) RecoveryVariantNeeded(ctx android.BaseModuleContext) bool {
 	return Bool(g.Recovery_available)
 }