Add vendor-ramdisk image to Soong.

Add vendor_ramdisk_available and vendor_ramdisk attribute to
various rules. When a vendor_ramdisk variant of a module is
generated, it is installed to $OUT/vendor-ramdisk.

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

Change-Id: Ib2d16459f3094dbe21c3bdb7c016cb4b2bf62765

rust/rust.go

@@ -118,6 +118,10 @@ func (mod *Module) RamdiskVariantNeeded(android.BaseModuleContext) bool {
 	return mod.InRamdisk()
 }
 
+func (mod *Module) VendorRamdiskVariantNeeded(android.BaseModuleContext) bool {
+	return mod.InVendorRamdisk()
+}
+
 func (mod *Module) RecoveryVariantNeeded(android.BaseModuleContext) bool {
 	return mod.InRecovery()
 }
@@ -185,6 +189,10 @@ func (mod *Module) OnlyInRamdisk() bool {
 	return false
 }
 
+func (mod *Module) OnlyInVendorRamdisk() bool {
+	return false
+}
+
 func (mod *Module) OnlyInRecovery() bool {
 	return false
 }