base-files: Add ubus events on sysupgrade
Event form: sysupgrade { "status": "", "bank_id": "<bank_id>"}
At the moment status can take 5 values, according to TR-181 Device.DeviceInfo.FirmwareImage: Active, Available, Downloading, InstallationFailed, ActivationFailed
Tested on MTK BPi, successful upgrade of flash bank 1 generates events in following order:
{ "sysupgrade": {"status":"Downloading","bank_id":"1"} }
{ "sysupgrade": {"status":"Available","bank_id":"1"} }
(reboot)
{ "sysupgrade": {"status":"Active","bank_id":"1"} }
{ "sysupgrade": {"status":"Available","bank_id":"2"} }