boot: zephyr: rework stm32h750b_dk board overlay & conf

Following the introduction of stm32h750b-dk 'ext_flash_app' board variant,
for storing apps in external Flash and chainloading them with MCUboot that's
placed in internal Flash,
we need a DT overlay to designate internal Flash & controller as the chosen
'zephyr,flash' & 'zephyr,flash-controller' for the MCUboot Zephyr Application.

BOOT_MAX_IMG_SECTORS_AUTO no longer need to be disabled after
including the 'soc-nv-flash' compatible in the board's external flash node,
& declaring the 'erase-block-size' & 'write-block-size' properties.

Signed-off-by: Abderrahmane JARMOUNI <git@jarmouni.me>

Author: JarmouniA

boot/zephyr/boards/stm32h750b_dk.conf

@@ -0,0 +1 @@
+CONFIG_STM32_MEMMAP=y

boot/zephyr/socs/stm32h750xx_ext_flash_app.conf

@@ -0,0 +1,7 @@
+/ {
+	chosen {
+		zephyr,flash = &flash0;
+		zephyr,flash-controller = &flash;
+		zephyr,code-partition = &boot_partition;
+	};
+};