Add host-side support for bootsel command

src/main.rs

@@ -213,6 +213,10 @@ fn main() -> Result<(), Error> {
                 }
                 println!();
             }
+            "bootsel" => {
+                writer.write_all(&[0x99])?;
+                writer.flush()?;
+            }
             _ => {
                 let slash_position = args[1].find("/").ok_or(Error {
                     message: "Error retrieving card info".to_owned(),