3748: TR-181 Device.DeviceInfo.FirmwareImage

This implements big parts of the Device.DeviceInfo.FirmwareImage interface. On a high level, this:

• improves sysupgrade to support checksums, in our case SHA-256 (as that is what we have available on the system), HTTP username/password-protected image downloading;
• exposes an upgrade() method on the fwbank ubus object, making use of the aforementioned sysupgrade functionalities;
• implements the Device.DeviceInfo.FirmwareImage.Status field and exposes it on fwbank.dump as a field for the bank JSON objects
• cleans up the ubus fwbank implementation
• cleans up parts of the iopsys-specific sysupgrade implementation

see also ticket #3748 on redmine.