simulate device specific repos via boardmappings

test repository filtering

Add unit tests for other ssu lib functions.

Member SsuDeviceInfo::variable (const QString &section, const QString &key)

compat-setting as ssudeviceinfo guaranteed to prepend sections with var-; SsuVariables does not have this guarantee. Remove from here as well.

Member SsuVariables::variable (const QString &section, const QString &key)

add override capability with an override-section in ssu.ini