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.serial_recovery b/boot/zephyr/Kconfig.serial_recovery
index 61fe58b..eabebfa 100644
--- a/boot/zephyr/Kconfig.serial_recovery
+++ b/boot/zephyr/Kconfig.serial_recovery
@@ -70,6 +70,7 @@
 	default 13 if BOARD_NRF52DK_NRF52832 || BOARD_NRF52833DK_NRF52833
 	default 23 if BOARD_NRF5340PDK_NRF5340_CPUAPP || BOARD_NRF5340PDK_NRF5340_CPUAPP_NS || \
 		BOARD_NRF5340DK_NRF5340_CPUAPP || BOARD_NRF5340DK_NRF5340_CPUAPP_NS
+	default -1
 	help
 	  Pin on the serial detect port that triggers serial recovery mode.