[TLS] Add support for platform-specific CA bundles.

Adds a new OS::get_system_ca_certs method which can be implemented by
platforms to retrieve the list of trusted CA certificates using OS
specific APIs.

The function should return the certificates in PEM format, and is
currently implemented for Windows/macOS/LinuxBSD(*)/Android.

mbedTLS will fall back to bundled certificates when the OS returns no
certificates.

(*) LinuxBSD does not have a standardized certificates store location.
    The current implementation will test for common locations and may
    return an empty string on some distributions (falling back to the
    bundled certificates).

SConstruct

@@ -217,7 +217,11 @@ opts.Add(BoolVariable("disable_advanced_gui", "Disable advanced GUI nodes and be
 opts.Add("build_profile", "Path to a file containing a feature build profile", "")
 opts.Add(BoolVariable("modules_enabled_by_default", "If no, disable all modules except ones explicitly enabled", True))
 opts.Add(BoolVariable("no_editor_splash", "Don't use the custom splash screen for the editor", True))
-opts.Add("system_certs_path", "Use this path as SSL certificates default for editor (for package maintainers)", "")
+opts.Add(
+    "system_certs_path",
+    "Use this path as TLS certificates default for editor and Linux/BSD export templates (for package maintainers)",
+    "",
+)
 opts.Add(BoolVariable("use_precise_math_checks", "Math checks use very precise epsilon (debug option)", False))
 
 # Thirdparty libraries