boards: Make the watchdog more aggressive

Let's not tune for debug by default... the point of the bootloader is that
it is for use when the SWD pins are not easily accessed.

src/boards.c

@@ -484,7 +484,9 @@ void neopixel_write (uint8_t *pixels)
 void wdt_init(void)
 {
   // 1 => keep running during a sleep, stop during SWD debug
-  nrf_wdt_behaviour_set(NRF_WDT, 1);
+  // 9 => run during sleep and SWD debug (e.g. most robust but
+  //      makes debugger attachment difficult)
+  nrf_wdt_behaviour_set(NRF_WDT, 9);
 
   // timeout after 5 seconds
   nrf_wdt_reload_value_set(NRF_WDT, 5 * 32768);