global-functions: $CertificateAvailable: try to use builtin certificates

The builtin certificates were introduced with RouterOS 7.19, so requires
this hacky :parse workaround.

README.md

@@ -72,7 +72,15 @@ including demonstation recorded live at [MUM Europe
 ### The long way in detail
 
 The update script does server certificate verification, so first step is to
-download the certificates. If you intend to download the scripts from a
+download the certificates.
+
+> 💡️ **Hint**: RouterOS 7.19 comes with a builtin certificate store. You
+> can skip the steps regarding certificate download and import and jump
+> to [installation of scripts](#installation-of-scripts) if you set the
+> trust for these builtin trust anchors:
+> `/certificate/settings/set builtin-trust-anchors=trusted;`
+
+If you intend to download the scripts from a
 different location (for example from github.com) install the corresponding
 certificate chain.
 
@@ -106,6 +114,8 @@ is shown.
 
 Always make sure there are no certificates installed you do not know or want!
 
+#### Installation of scripts
+
 All following commands will verify the server certificate. For validity the
 certificate's lifetime is checked with local time, so make sure the device's
 date and time is set correctly!

global-functions.rsc

@@ -119,6 +119,11 @@
     :return false;
   }
 
+  :if (([ /certificate/settings/get ]->"builtin-trust-anchors") = "trusted" && \
+       [[ :parse (":return [ :len [ /certificate/builtin/find where common-name=\"" . $CommonName . "\" ] ]") ]] > 0) do={
+    :return true;
+  }
+
   :if ([ :len [ /certificate/find where common-name=$CommonName ] ] = 0) do={
     $LogPrint info $0 ("Certificate with CommonName '" . $CommonName . "' not available.");
     :if ([ $CertificateDownload $CommonName ] = false) do={