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
diff --git a/boot/zephyr/boards/nrf52840_pca10056_big.overlay b/boot/zephyr/boards/nrf52840_big.overlay
similarity index 100%
rename from boot/zephyr/boards/nrf52840_pca10056_big.overlay
rename to boot/zephyr/boards/nrf52840_big.overlay
diff --git a/boot/zephyr/sample.yaml b/boot/zephyr/sample.yaml
index 62518c1..548b30b 100644
--- a/boot/zephyr/sample.yaml
+++ b/boot/zephyr/sample.yaml
@@ -5,16 +5,16 @@
 tests:
   sample.bootloader.mcuboot:
     tags: bootloader_mcuboot
-    platform_whitelist: nrf52840_pca10056 frdm_k64f
+    platform_whitelist: nrf52840dk_nrf52840 frdm_k64f
   sample.bootloader.mcuboot.serial_recovery:
     extra_args: OVERLAY_CONFIG=serial_recovery.conf
-    platform_whitelist:  nrf52840_pca10056
+    platform_whitelist:  nrf52840dk_nrf52840
     tags: bootloader_mcuboot
   sample.bootloader.mcuboot.usb_cdc_acm_recovery:
     tags: bootloader_mcuboot
     platform_whitelist:  nrf52840_pca10059
   sample.bootloader.mcuboot.usb_cdc_acm_recovery_log:
     extra_args: OVERLAY_CONFIG=./usb_cdc_acm_log_recovery.conf
-      DTC_OVERLAY=./boards/nrf52840_pca10056_big.overlay
-    platform_whitelist:  nrf52840_pca10056
+      DTC_OVERLAY=./boards/nrf52840_big.overlay
+    platform_whitelist:  nrf52840dk_nrf52840
     tags: bootloader_mcuboot
diff --git a/ext/nrf/README.md b/ext/nrf/README.md
index a22f06a..ba4cfaa 100644
--- a/ext/nrf/README.md
+++ b/ext/nrf/README.md
@@ -11,7 +11,7 @@
 
 ```
 mkdir build && cd build
-cmake -GNinja -DBOARD=nrf52840_pca10056
+cmake -GNinja -DBOARD=nrf52840dk_nrf52840
 ninja flash
 ```