boot: zephyr: Add default values of boot detect pins

Change introduces default values of CONFIG_BOOT_SERIAL_DETECT_PIN
and CONFIG_BOOT_USB_DFU_DETECT_PIN. This is needed to prevent build
issues caused by uninitialized Kconfig.

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index cbdbd59..3604964 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -448,6 +448,7 @@
 	default 13 if BOARD_NRF52DK_NRF52832
 	default 23 if BOARD_NRF5340_DK_NRF5340_CPUAPP || BOARD_NRF5340_DK_NRF5340_CPUAPP_NS
 	default 43 if BOARD_BL5340_DVK_CPUAPP || BOARD_BL5340_DVK_CPUAPP_NS
+	default -1
 	help
 	  Pin on the DFU detect port that triggers DFU mode.