test/le_audio: added documentation

test/le_audio/README.md

@@ -0,0 +1,49 @@
+These samples are meant to be used with the [PTS Dongel](https://bluekitchen-gmbh.com/bluetooth-pts-with-nordic-nrf52840-usb-dongle/) or an equivalent setup like
+the one described in here using a nrf5340dk.
+
+# nrf5340dk as HCI dongle
+for this a working [Zephyr](https://www.zephyrproject.org/) build environment is required, where the setup of Zephyr is beyond the scope of this document.
+
+The nrf5340 is a dual core SOC for which the network core handles the low level radio control and the application core handles the actual application.
+So to make a working dongle the network core and the application core need to be programmed.
+
+### network core / Packetcraft LL
+for nrf5340 the latest netcore firmware is located at [sdk-nrf](https://github.com/nrfconnect/sdk-nrf/tree/main/lib/bin/bt_ll_acs_nrf53/bin)
+to program it:
+```sh
+nrfjprog --program ble5-ctr-rpmsg_<version number>.hex --chiperase --coprocessor CP_NETWORK -r
+```
+
+### application core
+the `hci_uart` sample is used here over USB CDC
+build using:
+```sh
+west build -b nrf5340dk_nrf5340_cpuapp -- -DDTC_OVERLAY_FILE=usb.overlay -DOVERLAY_CONFIG=overlay-usb.conf
+```
+with `usb.overlay` specifying to use USB CDC instead of a physical UART
+```c
+/*
+ * Copyright (c) 2021 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+ */
+
+/ {
+	chosen {
+		zephyr,bt-c2h-uart = &cdc_acm_uart0;
+	};
+};
+
+&zephyr_udc0 {
+	cdc_acm_uart0: cdc_acm_uart0 {
+		compatible = "zephyr,cdc-acm-uart";
+	};
+};
+```
+and `overlay-usb.conf` to enable USB
+```make
+CONFIG_USB_DEVICE_STACK=y
+CONFIG_USB_DEVICE_PRODUCT="Zephyr HCI UART sample"
+CONFIG_USB_CDC_ACM=y
+CONFIG_USB_DEVICE_INITIALIZE_AT_BOOT=n
+```