zephyr:boards nrf52840_pca10056 rename to nrf52840dk_nrf52840

Board was renamed here:
https://github.com/zephyrproject-rtos/zephyr/pull/23524/

Patch aligns the name in MCUBoot and decouples one of nrf52840 dedicated
overlay from exact board name.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index 591a43b..31b6db3 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -325,7 +325,7 @@
 config BOOT_SERIAL_DETECT_PIN
 	int "Pin to trigger serial recovery mode"
 	default 6 if BOARD_NRF9160_PCA10090
-	default 11 if BOARD_NRF52840_PCA10056
+	default 11 if BOARD_NRF52840DK_NRF52840
 	default 13 if BOARD_NRF52_PCA10040
 	default 23 if BOARD_NRF5340_DK_NRF5340_CPUAPP || BOARD_NRF5340_DK_NRF5340_CPUAPPNS
 	help