RemoteLinux: Declare step id constants publicly

... so the steps can be referred to by Id only in some places in
Qdb (and Qnx later), thus weakening the hard compile-time dependencies
between the plugins.

Change-Id: Ia3000b5044a8d6eb29a424a05bccb88a4c6126ab
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/remotelinux/makeinstallstep.cpp

@@ -25,6 +25,8 @@
 
 #include "makeinstallstep.h"
 
+#include "remotelinux_constants.h"
+
 #include <projectexplorer/buildconfiguration.h>
 #include <projectexplorer/buildsteplist.h>
 #include <projectexplorer/buildsystem.h>
@@ -112,7 +114,7 @@ MakeInstallStep::MakeInstallStep(BuildStepList *parent, Core::Id id) : MakeStep(
 
 Core::Id MakeInstallStep::stepId()
 {
-    return "RemoteLinux.MakeInstall";
+    return Constants::MakeInstallStepId;
 }
 
 QString MakeInstallStep::displayName()