Add ramdisk image.

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

Change-Id: I11389777c6bfb0c0d73bbb4c70091c1e70f44077
diff --git a/android/image.go b/android/image.go
index 5291ce3..061bfa5 100644
--- a/android/image.go
+++ b/android/image.go
@@ -22,6 +22,10 @@
 	// CoreVariantNeeded should return true if the module needs a core variant (installed on the system image).
 	CoreVariantNeeded(ctx BaseModuleContext) bool
 
+	// RamdiskVariantNeeded should return true if the module needs a ramdisk variant (installed on the
+	// ramdisk partition).
+	RamdiskVariantNeeded(ctx BaseModuleContext) bool
+
 	// RecoveryVariantNeeded should return true if the module needs a recovery variant (installed on the
 	// recovery partition).
 	RecoveryVariantNeeded(ctx BaseModuleContext) bool
@@ -46,6 +50,9 @@
 
 	// RecoveryVariation means a module to be installed to recovery image.
 	RecoveryVariation string = "recovery"
+
+	// RamdiskVariation means a module to be installed to ramdisk image.
+	RamdiskVariation string = "ramdisk"
 )
 
 // imageMutator creates variants for modules that implement the ImageInterface that
@@ -63,6 +70,9 @@
 		if m.CoreVariantNeeded(ctx) {
 			variations = append(variations, CoreVariation)
 		}
+		if m.RamdiskVariantNeeded(ctx) {
+			variations = append(variations, RamdiskVariation)
+		}
 		if m.RecoveryVariantNeeded(ctx) {
 			variations = append(variations, RecoveryVariation)
 		}