bootloader: bootloader start is triggered by magic number

To improve stability if the sketch overwrite the last SRAM location
(in this case the double tap doesn't work but at least the sketch
starts without problems)

bootloaders/zero/main.c

@@ -98,6 +98,7 @@ static void check_start_application(void)
 	led_port->OUTCLR.reg = (1<<30);
 
 #if defined(BOOT_DOUBLE_TAP_ADDRESS)
+	#define DOUBLE_TAP_MAGIC 0x07738135
 	if (PM->RCAUSE.bit.POR)
 	{
 		/* On power-on initialize double-tap */
@@ -105,9 +106,10 @@ static void check_start_application(void)
 	}
 	else
 	{
-		if (BOOT_DOUBLE_TAP_DATA == 0) {
+		switch (BOOT_DOUBLE_TAP_DATA) {
+		case 0:
 			/* First tap */
-			BOOT_DOUBLE_TAP_DATA = 1;
+			BOOT_DOUBLE_TAP_DATA = DOUBLE_TAP_MAGIC;
 
 			for (uint32_t i=0; i<50000; i++) /* 200ms */
 				/* force compiler to not optimize this... */
@@ -115,10 +117,14 @@ static void check_start_application(void)
 
 			/* Timeout happened, continue boot... */
 			BOOT_DOUBLE_TAP_DATA = 0;
-		} else {
+			break;
+		case DOUBLE_TAP_MAGIC:
 			/* Second tap, stay in bootloader */
 			BOOT_DOUBLE_TAP_DATA = 0;
 			return;
+		default:
+			/* Fallback... reset counter and continue boot */
+			BOOT_DOUBLE_TAP_DATA = 0;
 		}
 	}
 #endif