HACK: main: Increase time spent in UART DFU mode

This is useful for devices that cannot be reset from the UART control
signals... a one second interval is too hard to coordinate.

src/main.c

@@ -109,7 +109,7 @@ void usb_teardown(void);
 #define DFU_DBL_RESET_MEM               0x20007F7C
 
 #define BOOTLOADER_VERSION_REGISTER     NRF_TIMER2->CC[0]
-#define DFU_SERIAL_STARTUP_INTERVAL     1000
+#define DFU_SERIAL_STARTUP_INTERVAL     3000
 
 // These value must be the same with one in dfu_transport_ble.c
 #define BLEGAP_EVENT_LENGTH             6