base-files: sysupgrade: include uci-defaults script disabling services

base-files: sysupgrade: include uci-defaults script disabling services

Disabled services should be kept disabled after sysupgrade. This can be
easily handled using a proper uci-defaults script.

Extend sysupgrade to check for disabled services, generate uci-defaults
script disabling them and include it in backup.

Cc: Christian Marangi <[email protected]>
Cc: Jo-Philipp Wich <[email protected]>
Cc: Jonas Gorski <[email protected]>
Signed-off-by: Rafał Miłecki <[email protected]>
Acked-by: Jo-Philipp Wich <[email protected]>