Linux embedded: misc improvements

- Allow static build
- Use .svg icons
- Linux Embedded lacks normal "open file dialog". So simply scan
  files in root folder of USB stick and return list.
- Change QProcess::execute(command) calls to
  QProcess::execute(command, args) calls to silence Qt 5.15.0
  depreciation warning.

driveformatthread.cpp

@@ -186,7 +186,7 @@ void DriveFormatThread::run()
         return;
     }
 
-    proc.execute("partprobe");
+    proc.execute("partprobe", QStringList() );
 
     args.clear();
     args << fatpartition;