make the puzzle dongle app operate on channel 25

and adjust puzzle-radio starter code accordingly

this reduces the chance of people using puzzle-radio with the wrong dongle
app (i.e. loopback); that would lead to them getting response different than
what's indicated in the text

I have also added the app name to the dongle / serial-term output:

``` text
deviceid=75fa82ad7588b3c7 channel=20 TxPower=+8dBm app=loopback.hex
```

beginner/apps/src/bin/radio-puzzle.rs

@@ -5,7 +5,7 @@
 use core::str;
 
 use cortex_m_rt::entry;
-use dk::ieee802154::Packet;
+use dk::ieee802154::{Channel, Packet};
 use panic_log as _; // the panicking behavior
 
 #[entry]
@@ -13,6 +13,9 @@ fn main() -> ! {
     let board = dk::init().unwrap();
     let mut radio = board.radio;
 
+    // puzzle.hex uses channel 25
+    radio.set_channel(Channel::_25);
+
     let mut packet = Packet::new();
 
     // try these