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/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