boot: zephyr: Add Cortex-R support

Add Cortex-R support by using ifdefs to change the vector table, remove
CORTEX_M only code and do an explicit bx instruction to switch from
Thumb to ARM mode on exit of the MCUboot application.

Signed-off-by: Mika Braunschweig <mika.braunschweig@siemens.com>

Author: m-braunschweig

boot/zephyr/main.c

@@ -3,6 +3,7 @@
  * Copyright (c) 2020 Arm Limited
  * Copyright (c) 2021-2023 Nordic Semiconductor ASA
  * Copyright (c) 2025 Aerlync Labs Inc.
+ * Copyright (c) 2025 Siemens Mobility GmbH
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,7 +33,7 @@
 #include <zephyr/cache.h>
 #endif
 
-#if defined(CONFIG_ARM)
+#if defined(CONFIG_CPU_CORTEX_M)
 #include <cmsis_core.h>
 #endif
 
@@ -142,8 +143,19 @@ extern void *_vector_table_pointer;
 #endif
 
 struct arm_vector_table {
+#ifdef CONFIG_CPU_CORTEX_M
     uint32_t msp;
     uint32_t reset;
+#else
+    uint32_t reset;
+    uint32_t undef_instruction;
+    uint32_t svc;
+    uint32_t abort_prefetch;
+    uint32_t abort_data;
+    uint32_t reserved;
+    uint32_t irq;
+    uint32_t fiq;
+#endif
 };
 
 static void do_boot(struct boot_rsp *rsp)
@@ -226,7 +238,10 @@ static void do_boot(struct boot_rsp *rsp)
 #endif
 #endif /* CONFIG_BOOT_INTR_VEC_RELOC */
 
+#ifdef CONFIG_CPU_CORTEX_M
     __set_MSP(vt->msp);
+#endif
+
 #if CONFIG_MCUBOOT_CLEANUP_ARM_CORE
     __set_CONTROL(0x00); /* application will configures core on its own */
     __ISB();
@@ -257,7 +272,17 @@ static void do_boot(struct boot_rsp *rsp)
         : "r0", "r1", "r2", "r3", "memory"
     );
 #else
+
+#ifdef CONFIG_CPU_CORTEX_M
     ((void (*)(void))vt->reset)();
+#else
+    /* Some architectures like the Cortex-R run in thumb mode but reset into ARM
+     * mode. To do the switch into ARM mode an explicit branch with exchange
+     * instruction set is needed
+     */
+    __asm__("bx %0\n" : : "r" (&vt->reset));
+#endif
+
 #endif
 }