copyUbootFiles: don't continue if deviceinfo_ubootboardname is unset (MR 18)

If this isn't set in the deviceinfo file, then the device probably
doesn't support the post-processing-with-uboot-files stuff that c07eafd0
set out to address.

main.go

@@ -598,6 +598,10 @@ func Copy(srcFile, dstFile string) error {
 }
 
 func copyUbootFiles(path string, devinfo deviceinfo.DeviceInfo) error {
+	if devinfo.UbootBoardname == "" {
+		return nil
+	}
+
 	srcDir := filepath.Join("/usr/share/u-boot", devinfo.UbootBoardname)
 	entries, err := ioutil.ReadDir(srcDir)
 	if err != nil {