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

cc/linkable.go

@@ -40,6 +40,9 @@ type LinkableInterface interface {
 	InRamdisk() bool
 	OnlyInRamdisk() bool
 
+	InVendorRamdisk() bool
+	OnlyInVendorRamdisk() bool
+
 	InRecovery() bool
 	OnlyInRecovery() bool