Add ramdisk image.

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

Change-Id: I11389777c6bfb0c0d73bbb4c70091c1e70f44077
diff --git a/cc/linkable.go b/cc/linkable.go
index 106092b..3c46d9d 100644
--- a/cc/linkable.go
+++ b/cc/linkable.go
@@ -38,6 +38,9 @@
 	Shared() bool
 	Toc() android.OptionalPath
 
+	InRamdisk() bool
+	OnlyInRamdisk() bool
+
 	InRecovery() bool
 	OnlyInRecovery() bool