tools: ynl: allow user to specify flag attr with bool values

The flag attr presence in Netlink message indicates value "true",
if it is missing in the message it means "false".

Allow user to specify attrname with value "true"/"false"
in json for flag attrs, treat "false" value properly.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240222134351.224704-2-jiri@resnulli.usSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/ynl.py

@@ -459,6 +459,9 @@ class YnlFamily(SpecFamily):
                 attr_payload += self._add_attr(attr['nested-attributes'],
                                                subname, subvalue, sub_attrs)
         elif attr["type"] == 'flag':
+            if not value:
+                # If value is absent or false then skip attribute creation.
+                return b''
             attr_payload = b''
         elif attr["type"] == 'string':
             attr_payload = str(value).encode('ascii') + b'\x00'