gui: Use symlink for config.json

- Cargo package does not like files being in root workspace directory.
  So use symlink

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

gui/config.json

+../config.json
\ No newline at end of file

gui/src/constants.rs

 pub const BB_IMAGER_ORIGINAL_CONFIG: &str = "https://dl.beagle.cc/distros.json";
-pub const DEFAULT_CONFIG: &[u8] = include_bytes!("../../config.json");
+pub const DEFAULT_CONFIG: &[u8] = include_bytes!("../config.json");
 pub const APP_NAME: &str = "BeagleBoard Imager";
 
 pub const WINDOW_ICON: &[u8] = include_bytes!("../icon.png");