vector.h: Add slot for openblt signature

The openblt boot loader needs one extra slot in the vector table in
order to store the signature and detect a valid boot image.

include/libopencm3/cm3/vector.h

@@ -59,6 +59,8 @@ typedef struct {
 	vector_table_entry_t pend_sv;
 	vector_table_entry_t systick;
 	vector_table_entry_t irq[NVIC_IRQ_COUNT];
+	/* https://www.feaser.com/openblt/doku.php?id=manual:ports:armcm3_stm32 */
+	vector_table_entry_t openblt_signature;
 } vector_table_t;
 
 /* Common symbols exported by the linker script(s): */