stages: fix ostree config stage

Currently we're using `str(value)` on a boolean which yields `True` or
`False` - turns out ostree reads these values case sensitive and despite
setting `True|False`, it doesn't just work.

From jlebon on slack:

> the syntax is readonly=true . it's case sensitive

Fix the above and also just remove the loop as, while it's handy, we'll
have to differentiate between options' values anyway and it's just two
options we support today.

Signed-off-by: Antonio Murdaca <runcom@linux.com>

stages/org.osbuild.ostree.config

@@ -63,13 +63,16 @@ def ostree(*args, _input=None, **kwargs):
 
 def main(tree, options):
     repo = os.path.join(tree, options["repo"].lstrip("/"))
-    config = options["config"]
+    sysroot_options = options["config"].get("sysroot", {})
 
-    for group, items in config.items():
-        for key, value in items.items():
-            name = f"{group}.{key}"
-            ostree("config", "set", name, str(value),
-                   repo=repo)
+    bootloader = sysroot_options.get("bootloader")
+    if bootloader:
+        ostree("config", "set", "sysroot.bootloader", bootloader, repo=repo)
+
+    readonly = sysroot_options.get("readonly")
+    if readonly is not None:  # can be False, which we would want to set
+        ro = "true" if readonly else "false"
+        ostree("config", "set", "sysroot.readonly", ro, repo=repo)
 
 
 if __name__ == '__main__':