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 <ansuelsmth@gmail.com>
Cc: Jo-Philipp Wich <jo@mein.io>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Jo-Philipp Wich <jo@mein.io>