Enable OTA DFU mode

I'm using precisely this as it is what most smart watches do to enter bootloder, so something like this could work

+++ b/targets/nrf5x_dfu/main.c
@@ -100,13 +100,39 @@ bool dfu_enter_check(void) {
 [#else](https://forum.espruino.com/search/?q=%23else)
   dfu_start = dfuIsColdBoot; // if no button, enter bootloader if it's a cold boot, then exit after a few seconds
 [#endif](https://forum.espruino.com/search/?q=%23endif)
+  if (NRF_POWER->GPREGRET == 1) { NRF_POWER->GPREGRET=0; return true; }
[#ifdef](https://forum.espruino.com/search/?q=%23ifdef) BUTTONPRESS_TO_REBOOT_BOOTLOADER^M
     // if DFU looks invalid, go straight to bootloader
     if (s_dfu_settings.bank_0.bank_code == NRF_DFU_BANK_INVALID) {
       lcd_println("BANK0 INVALID");

so just one line to put here https://github.com/espruino/Espruino/blob/master/targets/nrf5x_dfu/main.c#L103

and then just poke32(0x4000051c,1) triggers DFU. However as Gordon mentions it is a bit dangerous, anyone can trigger DFU update remotely if you don't set up console password or other protection (like whitelisting/refusing unknown connections)