zephyr: remove deprecated DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL

DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL is going to be deprecated, so
remove its usages from the Zephyr port. Definition checks have been
replaced with DT_HAS_CHOSEN(zephyr_flash_controller), and the macro
itself has been replaced by
DT_LABEL(DT_CHOSEN(zephyr_flash_controller)). Note that the code could
likely be refactored to make use of compile time device references, ie
use DEVICE_DT_GET, but that task has been left for the maintainers.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index a52d58f..c5832cb 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -6,6 +6,7 @@
  */
 
 #include <zephyr.h>
+#include <devicetree.h>
 #include <drivers/flash.h>
 
 #include "target.h"
@@ -17,7 +18,7 @@
 
 BOOT_LOG_MODULE_DECLARE(mcuboot);
 
-#if (!defined(CONFIG_XTENSA) && defined(DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL))
+#if (!defined(CONFIG_XTENSA) && DT_HAS_CHOSEN(zephyr_flash_controller))
 #define FLASH_DEVICE_ID SOC_FLASH_0_ID
 #define FLASH_DEVICE_BASE CONFIG_FLASH_BASE_ADDRESS
 #elif (defined(CONFIG_XTENSA) && defined(DT_JEDEC_SPI_NOR_0_LABEL))