bsps/arm: Add BSP_START_VECTOR_RESERVED_SLOT

Allow BSPs to customize the reserved vector table slot.  By default, use
an undefined instruction.

bsps/arm/shared/start/start.S

@@ -71,8 +71,11 @@ bsp_start_vector_table_begin:
 	ldr	pc, .Lhandler_addr_prefetch
 	ldr	pc, .Lhandler_addr_abort
 
-	/* Program signature checked by boot loader */
-	.word	0xb8a06f58
+#ifdef BSP_START_VECTOR_RESERVED_SLOT
+	BSP_START_VECTOR_RESERVED_SLOT
+#else
+	udf
+#endif
 
 	ldr	pc, .Lhandler_addr_irq
 	ldr	pc, .Lhandler_addr_fiq

spec/build/bsps/arm/optstartvecresvslot.yml

@@ -0,0 +1,18 @@
+SPDX-License-Identifier: CC-BY-SA-4.0 OR BSD-2-Clause
+actions:
+- get-integer: null
+- format-and-define: null
+build-type: option
+copyrights:
+- Copyright (C) 2024 embedded brains GmbH & Co. KG
+default:
+- enabled-by: bsps/arm/lpc24xx
+  value: .word 0xb8a06f58
+description: |
+  Defines the content of the reserved slot in the vector table.  Some targets
+  use it for a boot loader signature or checksum of the vector table entries.
+enabled-by: true
+format: '{}'
+links: []
+name: BSP_START_VECTOR_RESERVED_SLOT
+type: build

spec/build/bsps/arm/start.yml

@@ -10,6 +10,8 @@ install-path: ${BSP_LIBDIR}
 links:
 - role: build-dependency
   uid: optstartvecaddralign
+- role: build-dependency
+  uid: optstartvecresvslot
 source:
 - bsps/arm/shared/start/start.S
 target: start.o