helper/config: copy buildname/buildtype properly

helper/config/decode.go

@@ -42,6 +42,8 @@ func Decode(target interface{}, config *DecodeOpts, raws ...interface{}) error {
 		if config.InterpolateContext == nil {
 			config.InterpolateContext = ctx
 		} else {
+			config.InterpolateContext.BuildName = ctx.BuildName
+			config.InterpolateContext.BuildType = ctx.BuildType
 			config.InterpolateContext.TemplatePath = ctx.TemplatePath
 			config.InterpolateContext.UserVariables = ctx.UserVariables
 		}

template/interpolate/funcs.go

@@ -71,7 +71,7 @@ func funcGenBuildName(ctx *Context) interface{} {
 func funcGenBuildType(ctx *Context) interface{} {
 	return func() (string, error) {
 		if ctx == nil || ctx.BuildType == "" {
-			return "", errors.New("build_name not available")
+			return "", errors.New("build_type not available")
 		}
 
 		return ctx.BuildType, nil