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
diff --git a/genrule/genrule.go b/genrule/genrule.go
index 99d6207..b09c195 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -605,6 +605,7 @@
 func (x noopImageInterface) ImageMutatorBegin(android.BaseModuleContext)                 {}
 func (x noopImageInterface) CoreVariantNeeded(android.BaseModuleContext) bool            { return false }
 func (x noopImageInterface) RamdiskVariantNeeded(android.BaseModuleContext) bool         { return false }
+func (x noopImageInterface) VendorRamdiskVariantNeeded(android.BaseModuleContext) bool   { return false }
 func (x noopImageInterface) RecoveryVariantNeeded(android.BaseModuleContext) bool        { return false }
 func (x noopImageInterface) ExtraImageVariations(ctx android.BaseModuleContext) []string { return nil }
 func (x noopImageInterface) SetImageVariation(ctx android.BaseModuleContext, variation string, module android.Module) {