stages/ignition: parametrize the path to boot

Allow passing a mount to specify where to write the igntion.firstboot
file.
This keeps the default `tree:///` value to not break existing stages.

stages/org.osbuild.ignition

@@ -2,9 +2,12 @@
 import sys
 
 import osbuild.api
+from osbuild.util import parsing
 
 
-def main(tree, options):
+def main(args, options):
+    target = options.get("target", "tree:///boot")
+    location = parsing.parse_location(target, args)
     network = options.get("network", [])
 
     # grub, when detecting the '/boot/ignition.firstboot' file
@@ -13,7 +16,7 @@ def main(tree, options):
     # itself will be sourced this the 'ignition_network_kcmdline'
     # that is also in the "ignition_firstboot" variable can be
     # overwritten with the contents of `network`
-    with open(f"{tree}/boot/ignition.firstboot", "w", encoding="utf8") as f:
+    with open(f"{location}/ignition.firstboot", "w", encoding="utf8") as f:
         if network:
             netstr = " ".join(network)
             f.write(f"set ignition_network_kcmdline='{netstr}'")
@@ -22,6 +25,6 @@ def main(tree, options):
 
 
 if __name__ == '__main__':
-    args = osbuild.api.arguments()
-    r = main(args["tree"], args.get("options", {}))
+    _args = osbuild.api.arguments()
+    r = main(_args, _args["options"])
     sys.exit(r)

stages/org.osbuild.ignition.meta.json

@@ -22,6 +22,11 @@
         "items": {
           "type": "string"
         }
+      },
+      "target": {
+        "type": "string",
+        "description": "Location to write the 'ignition.firstboot' file.",
+        "default": "tree:///boot"
       }
     }
   }