RemoteLinux: Use a RemoteLinux.SupportsRSync key

... into the Device::extraData map instead of dedicated accessors.

This weakens the compile time dependencies of the RSyncBuildStep,
LinuxDevice, and GenericLinuxDeviceTester at the price of weaker,
local typesafety for one bool value.

Potentially this could go into the direction of re-using bits
of "device feature" discovery and use. Nothing planned, though.

Change-Id: I9aa6dce8066b82d344f64f38707f17416e273957
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/remotelinux/linuxdevicetester.cpp

@@ -25,7 +25,7 @@
 
 #include "linuxdevicetester.h"
 
-#include "linuxdevice.h"
+#include "remotelinux_constants.h"
 #include "rsyncdeploystep.h"
 
 #include <projectexplorer/devicesupport/deviceusedportsgatherer.h>
@@ -261,7 +261,7 @@ void GenericLinuxDeviceTester::handleRsyncFinished()
         emit progressMessage(tr("rsync is functional.\n"));
     }
 
-    d->deviceConfiguration.staticCast<LinuxDevice>()->setSupportsRsync(error.isEmpty());
+    d->deviceConfiguration->setExtraData(Constants::SupportsRSync, error.isEmpty());
     setFinished(result);
 }