rbutil: enable "done" signal on bootloader uninstall

This allows to implement multi-stage uninstallers. Should not affect
the behaviour on current targets.

Change-Id: Idf8aec5caf76cf9317798890d094a7cebdbcabec

rbutil/rbutilqt/base/bootloaderinstallimx.cpp

@@ -175,6 +175,7 @@ bool BootloaderInstallImx::uninstall(void)
     emit logItem(tr("To uninstall, perform a normal upgrade with an unmodified "
                     "original firmware."), LOGINFO);
     logInstall(LogRemove);
+    emit done(true);
     return false;
 }