stm32cube: stm32n6: Allow building a chainloaded secure application

When chain loading a secure application after FSBL, some actions
should be avoided at reset.
Update _fsbl.c file to do this.
Prepare CMakeLists.txt if this should be done in _s.c file (which would
require configuring secure image which is not available yet).

Signed-off-by: Erwan Gouriou <[email protected]>

Author: erwango

stm32cube/stm32n6xx/CMakeLists.txt

@@ -1,8 +1,13 @@
 # Copyright (c) 2025 STMicroelectronics
 #
 # SPDX-License-Identifier: Apache-2.0
-
+#if(CONFIG_BOOTLOADER_MCUBOOT)
+#	zephyr_include_directories(soc)
+#	zephyr_include_directories(soc/Templates)
+#	zephyr_library_sources(soc/system_stm32n6xx_s.c)
+#else()
 zephyr_library_sources(soc/system_stm32n6xx_fsbl.c)
+#endif()
 zephyr_library_sources(drivers/src/stm32n6xx_hal.c)
 zephyr_library_sources(drivers/src/stm32n6xx_hal_rcc.c)
 zephyr_library_sources(drivers/src/stm32n6xx_hal_rcc_ex.c)

stm32cube/stm32n6xx/soc/system_stm32n6xx_fsbl.c

@@ -190,6 +190,7 @@ void SystemInit(void)
   SCB->VTOR = INTVECT_START;
 #endif  /* USER_VECT_TAB_ADDRESS */
 
+#if !defined(CONFIG_BOOTLOADER_MCUBOOT)
   /* RNG reset */
   RCC->AHB3RSTSR = RCC_AHB3RSTSR_RNGRSTS;
   RCC->AHB3RSTCR = RCC_AHB3RSTCR_RNGRSTC;
@@ -221,6 +222,7 @@ void SystemInit(void)
   SAU->RNR = 7;
   SAU->RBAR = 0;
   SAU->RLAR = 0;
+#endif
 
   /* System configuration setup */
   RCC->APB4ENSR2 = RCC_APB4ENSR2_SYSCFGENS;
@@ -230,6 +232,7 @@ void SystemInit(void)
   /* Set default Vector Table location after system reset or return from Standby */
   SYSCFG->INITSVTORCR = SCB->VTOR;
 
+#if !defined(CONFIG_BOOTLOADER_MCUBOOT)
   /* Enable VDDADC CLAMP */
   PWR->SVMCR3 |= PWR_SVMCR3_ASV;
   PWR->SVMCR3 |= PWR_SVMCR3_AVMEN;
@@ -255,6 +258,7 @@ void SystemInit(void)
 
   /* Deactivate GPIOG clock */
   RCC->AHB4ENCR = RCC_AHB4ENCR_GPIOGENC;
+#endif
 
   /* Read back the value to make sure it is written before deactivating SYSCFG */
   (void) SYSCFG->INITSVTORCR;