The shell sample, and a two-board rig for testing on hardware

samples/shell is `rb9704` on the shell with callbacks that print what
arrives, a CRC-32 and the start of each message from the network. Board
overlays for native_sim (the modem a host serial port, a pty or a real
9704 on USB), NUCLEO-U575ZI-Q, XIAO ESP32S3 and ESP32-S3-DevKitC.

tests/rig/bridge copies bytes between a board's USB serial port and a
UART, so two boards wired together stand in for a USB-serial adapter: one
runs the sample, the other bridges its modem UART to the host, where the
fake modem answers. Overlays for a Nucleo and a XIAO, either way round.

tests/rig/bridge/boards/nucleo_u575zi_q.overlay

@@ -0,0 +1,18 @@
+/* host: USART1, the ST-LINK's virtual COM port. target: USART2 on PD5/PD6,
+ * the CN9 UART block, wired to the other board's modem UART. Both at the
+ * modem's 230400. */
+/ {
+	aliases {
+		bridge-host = &usart1;
+		bridge-target = &usart2;
+	};
+};
+
+&usart1 {
+	current-speed = <230400>;
+};
+
+&usart2 {
+	current-speed = <230400>;
+	fifo-enable;
+};