STM32N6xx: add support for running from external flash

[Igor-Misic]

This is an example of IMLedBlink running from external flash in memory-mapped mode.

[Copilot]

Pull Request Overview

This PR adds support for running firmware from external flash on the STM32N6xx microcontroller in memory-mapped mode. It includes two new build targets and corresponding linker scripts to support both standard external flash execution and a variant with firmware signature support.

Key changes:

• Added two new build targets: nucleo_n657x0_q_ext and nucleo_n657x0_q_ram
• Created two new linker scripts for external flash configurations
• Disabled XSPI2/XSPIM peripheral reset during system initialization to preserve external flash configuration

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
Makefile	Added new build targets and rules for STM32N6xx external flash configuration
Linker/STM32N6xx_EXT_FLASH_SIGNATURE.ld	New linker script for external flash with firmware signature section
Linker/STM32N6xx_EXT_FLASH.ld	New linker script for external flash without signature section
LedBlink/STM32/Src/system_stm32n6xx.c	Commented out XSPI2/XSPIM reset to preserve external flash initialization

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