Extend the button blackout slightly

If the button blackout time is too short then we can reset before
we remove the splash screen... which means we can't hold down the
button when we transition to the payload... which, for wasp-os,
makes it really hard to trigger safe mode.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

src/boards.c

@@ -174,7 +174,7 @@ void SysTick_Handler(void)
    * recovering from a flat battery.
    */
   if (button_pressed(BUTTON_DFU)) {
-    if (_systick_count > 2000 && _long_press_count++ >  50) {
+    if (_systick_count > 2750 && _long_press_count++ >  50) {
       NRF_POWER->GPREGRET = BOARD_MAGIC_FORCE_APP_BOOT;
       NVIC_SystemReset();
     }