samples: sysbuild: with mcu_boot on the stm32 with octoSPI targets

Run the sample to execute in place on the external flash of the
b_u585i_iot02a or stm32h7b3i disco kit,
with_boot application is built and linked for the external flash

Signed-off-by: Francois Ramu <[email protected]>

Author: FRASTM

samples/sysbuild/with_mcuboot/boards/b_u585i_iot02a.conf

@@ -0,0 +1,2 @@
+CONFIG_FLASH=y
+

samples/sysbuild/with_mcuboot/boards/stm32h7b3i_dk.overlay

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2025 STMicroelectronics
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/*
+ * Define the device, controller and partition to be the external memory
+ * for running the application in external NOR from MCUboot
+ */
+
+/ {
+	chosen {
+		zephyr,flash = &mx25lm51245;
+		zephyr,flash-controller = &mx25lm51245;
+	};
+};