FMUv2: added bootloader delay signature to text

this allows for a configurable bootloader delay

nuttx-configs/px4fmu-v2/scripts/ld.script

@@ -66,12 +66,20 @@ EXTERN(_vectors)	/* force the vectors to be included in the output */
  * code pulled in by libgcc.a requires it (and that code cannot be easily avoided).
  */
 EXTERN(abort)
+EXTERN(_bootdelay_signature)
 
 SECTIONS
 {
 	.text : {
 		_stext = ABSOLUTE(.);
 		*(.vectors)
+                . = ALIGN(32);
+                /*
+                This signature provides the bootloader with a way to delay booting
+                */
+                _bootdelay_signature = ABSOLUTE(.);
+                FILL(0xffecc2925d7d05c5)
+                . += 8;
 		*(.text .text.*)        
 		*(.fixup)
 		*(.gnu.warning)