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/xiao_esp32s3_procpu.overlay

@@ -0,0 +1,33 @@
+/* host: the native USB serial port. target: uart1 on D8 (GPIO7, TX) and D9
+ * (GPIO8, RX), wired to the other board's modem UART. 230400, the modem's
+ * rate; the USB side has none. */
+/ {
+	aliases {
+		bridge-host = &usb_serial;
+		bridge-target = &uart1;
+	};
+};
+
+&spi2 {
+	status = "disabled";	/* the board puts it on GPIO7 to 9 */
+};
+
+&pinctrl {
+	uart1_bridge: uart1_bridge {
+		group1 {
+			pinmux = <UART1_TX_GPIO7>;
+			output-high;
+		};
+		group2 {
+			pinmux = <UART1_RX_GPIO8>;
+			bias-pull-up;
+		};
+	};
+};
+
+&uart1 {
+	pinctrl-0 = <&uart1_bridge>;
+	pinctrl-names = "default";
+	current-speed = <230400>;
+	status = "okay";
+};