Allow parameter override in URL lookup
Export basic set of repositories for a device
Add some device mode logic to ssu core
Add basic repository manager
Clear old settings before merging in new ones
Set correct device mode when changing flavour/rndRelease/release/
Fix several bugs related to repo management
Use only one instance of global settings, removing sync issues and unnecessary reads

Some bits got moved to more appropriate modules, others (like credentials
handling) still need to be moved. ssu CLI got a bit more readable.

This is part of a big refactoring due to SSU doing mostly repo management
and URL handling instead of the originally planned credentials management
with URL handling as a nice bonus feature.
Disable credentials handling for insecure repositories
Add repo management from CLI, build documentation
Enable repo listing from CLI
Move adaptation specific variable resolving into DeviceInfo
Drop obsolete adaptation and device key support
Add prototype of a kickstart generator
Do proper rnd/release handling; use lowercase name for searching partition files