stages/xorrisofs: ability to set system id

Add an option to set the systemd id of the ISO. The systemd id
is used by e.g. by osinfo to identify the RHEL boot isos, where
the system id is "LINUX"[1]

[1] https://gitlab.com/libosinfo/osinfo-db/-/blob/master/data/os/redhat.com/rhel-8.4.xml.in#L19

stages/org.osbuild.xorrisofs

@@ -36,6 +36,11 @@ SCHEMA_2 = r"""
       "description": "Volume id to set",
       "pattern": "[A-Z0-9_-]{1,32}"
     },
+    "sysid": {
+      "type": "string",
+      "description": "System id to set",
+      "pattern": "[A-Z0-9_-]{1,32}"
+    },
     "boot": {
       "type": "object",
       "additionalProperties": false,
@@ -85,6 +90,7 @@ def main(inputs, output_dir, options):
     boot = options.get("boot", {})
     filename = options["filename"]
     vol_id = options["volid"]
+    sys_id = options.get("sysid")
     efi = options.get("efi")
     isohybrid = options.get("isohybridmbr")
     isolevel = options.get("isolevel")
@@ -100,9 +106,14 @@ def main(inputs, output_dir, options):
         ]
 
     cmd += [
-        "-V", vol_id,
+        "-V", vol_id
     ]
 
+    if sys_id:
+        cmd += [
+            "-sysid", sys_id
+        ]
+
     if isohybrid:
         cmd += [
             "-isohybrid-mbr", isohybrid,