x509roots: fix generate script argument checking

Check for supply of both arguments forgot that the URL is set by
default. Instead just let the local path supersede the URL.

Change-Id: I0499137c99c735e8e453ff1c2a925435f3cd8039
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/504596
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>

Author: rolandshoemaker

x509roots/gen_fallback_bundle.go

@@ -64,18 +64,14 @@ var bundle = mustParse([]byte(pemRoots))
 `
 
 var (
-	certDataURL  = flag.String("certdata-url", "https://hg.mozilla.org/mozilla-central/raw-file/tip/security/nss/lib/ckfw/builtins/certdata.txt", "URL to the raw certdata.txt file to parse (only one of certdata-url and certdata-path may be specified)")
-	certDataPath = flag.String("certdata-path", "", "Path to the NSS certdata.txt file to parse (only one of certdata-url and certdata-path may be specified)")
+	certDataURL  = flag.String("certdata-url", "https://hg.mozilla.org/mozilla-central/raw-file/tip/security/nss/lib/ckfw/builtins/certdata.txt", "URL to the raw certdata.txt file to parse (certdata-path overrides this, if provided)")
+	certDataPath = flag.String("certdata-path", "", "Path to the NSS certdata.txt file to parse (this overrides certdata-url, if provided)")
 	output       = flag.String("output", "fallback/bundle.go", "Path to file to write output to")
 )
 
 func main() {
 	flag.Parse()
 
-	if *certDataPath != "" && *certDataURL != "" {
-		log.Fatal("Only one of --certdata-url and --certdata-path may be supplied")
-	}
-
 	var certdata io.Reader
 
 	if *certDataPath != "" {