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.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>