RemoteLinux: Use toMap() and fromMap() to serialize device info.

One more step towards extracting an abstract base device class.
Note that the device settings now go into a dedicated file, like
the Qt version and toolchain information, and no longer into the
QtCreator.ini file. Reading the "old style" settings is still
supported.

Change-Id: Ic4766420930c1ab650d72068d3f6d188aa9c203b
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>