strongswan: move strongswan ignore_routing_tables check into subfunction

strongswan: move strongswan ignore_routing_tables check into subfunction

This makes the generation process clearer and more extensible.

While we're at it, the option 'ignore_routing_tables' is changed to a list of
elements. For the migration, the '/etc/uci-defaults/strongswan' migration
script is be extended with the new function 'migrate_ignore_routing_tables',
which handles the migration from an 'option' element to a 'list' elements.

Signed-off-by: Florian Eckert <redacted>