RemoteLinux: Make mapping between targets and devices more flexible.

Until now, the mapping was based exclusively on the device type. The new
approach is more flexible and less static, allowing e.g. the set of
supported devices to depend on the current build settings.

Change-Id: Iec92bdf9fee647477b166f7624c2364a821e5cbd
Reviewed-by: Christian Kandeler <christian.kandeler@nokia.com>