samples: sensor: Use dedicated overlay for fake sensor in sensor_shell

Using an app.overlay file is inherently preventing the use of board
overlays. Switch to a dedicated overlay file so that e.g.
the vmu_rt1170/mimxrt1176/cm7 board target can pass this test where
before it would have failed due to its board-specific overlay "winning"
over the app.overlay file.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

samples/sensor/sensor_shell/Kconfig

@@ -3,6 +3,8 @@
 
 config SAMPLES_SENSOR_SHELL_FAKE_SENSOR
 	bool "Enable fake sensor"
+	default y
+	depends on DT_HAS_VND_FAKE_SENSOR_ENABLED
 	help
 	  On boards that do not have a sensor, enabling this will build a fake
 	  sensor that can be interacted with via the sensor shell.

samples/sensor/sensor_shell/README.rst

@@ -19,13 +19,14 @@ enabled, for example:
    :board: reel_board
    :goals: build flash
 
-For boards that do not have a sensor, a simple fake sensor driver is provided, for example:
+For boards that do not have a sensor, a simple fake sensor driver is provided and can enabled by
+adding the following overlay:
 
 .. zephyr-app-commands::
    :zephyr-app: samples/sensor/sensor_shell
    :board: qemu_riscv64
    :goals: run
-   :gen-args: -DCONFIG_SAMPLES_SENSOR_SHELL_FAKE_SENSOR=y
+   :gen-args: -DEXTRA_DTC_OVERLAY_FILE=fake_sensor.overlay
 
 Shell Module Command Help
 =========================

samples/sensor/sensor_shell/sample.yaml

@@ -22,6 +22,6 @@ tests:
     timeout: 180
     extra_configs:
       - arch:posix:CONFIG_NATIVE_UART_0_ON_STDINOUT=y
-      - CONFIG_SAMPLES_SENSOR_SHELL_FAKE_SENSOR=y
+    extra_args: DTC_OVERLAY_FILE=fake_sensor.overlay
     integration_platforms:
       - native_sim