samples: bluetooth: silabs: Bluetooth blinky

Similar to Silabs' SOC Blinky sample, implements two GATT characteristics.
One characteristic controls the state of the LED via write operations from
a GATT client. Second characteristic sends notifications to subscribed
clients when the button state changes on the board.
Minor changes have been made in README.rst, samples.yaml CMakeList.txt
Some coding style and global variable issues have been fixed.

Signed-off-by: Viktor Fisch <viktor.fisch@silabs.com>

Author: silabs-vifisch

samples/bt_blinky/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(bt_blinky)
+
+target_sources(app PRIVATE src/main.c)
+
+zephyr_library_include_directories(${ZEPHYR_BASE}/samples/bluetooth)

samples/bt_blinky/README.rst

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+.. zephyr:code-sample:: bt_blinky
+   :name: Blueooth blinky (Peripheral)
+   :relevant-api: bluetooth
+
+Overview
+********
+
+Similar to the SOC Blinky sample, implements a simple custom GATT service with two characteristics.
+One characteristic controls the state of the LED (ON/OFF) via write operations from a GATT client,
+and the second characteristic sends notifications to subscribed clients when the button state changes
+(pressed or released).
+
+Requirements
+************
+
+* BlueZ running on the host, or
+* A board with Bluetooth LE support
+
+Building and Running
+********************
+
+Building a minimal variant
+--------------------------
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/bt_blinky
+   :board: qemu_cortex_m3
+   :goals: build

samples/bt_blinky/prj.conf

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+CONFIG_BT=y
+CONFIG_LOG=y
+CONFIG_BT_SMP=y
+CONFIG_BT_PERIPHERAL=y
+CONFIG_BT_DIS=y
+CONFIG_BT_DIS_PNP=n
+CONFIG_BT_BAS=y
+CONFIG_BT_HRS=n
+CONFIG_BT_DEVICE_NAME="Blinky Example"
+CONFIG_BT_DEVICE_APPEARANCE=000

samples/bt_blinky/sample.yaml

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+sample:
+  name: Bluetooth Blinky
+  description: Similar to the Silabs' SOC Blinky sample, implements a simple custom
+              GATT service controlling the state of the LED (ON/OFF)
+              and sending notifications to subscribed clients when the button state changes.
+tests:
+  sample.bt_blinky:
+    harness: bluetooth
+    integration_platforms:
+      - xg24_dk2601b
+    tags:
+      - bluetooth
+      - led
+      - gpio
+    build_only: true