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itself has been replaced by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/include/target.h b/boot/zephyr/include/target.h
index 5f60daa..97365c2 100644
--- a/boot/zephyr/include/target.h
+++ b/boot/zephyr/include/target.h
@@ -19,6 +19,7 @@
  * Otherwise, the Zephyr SoC header and the DTS provide most
  * everything we need.
  */
+#include <devicetree.h>
 #include <soc.h>
 #include <storage/flash_map.h>
 
@@ -33,7 +34,7 @@
 /*
  * Sanity check the target support.
  */
-#if (!defined(CONFIG_XTENSA) && !defined(DT_CHOSEN_ZEPHYR_FLASH_CONTROLLER_LABEL)) || \
+#if (!defined(CONFIG_XTENSA) && !DT_HAS_CHOSEN(zephyr_flash_controller)) || \
     (defined(CONFIG_XTENSA) && !defined(JEDEC_SPI_NOR_0_LABEL)) || \
     !defined(FLASH_ALIGN) ||                  \
     !(FLASH_AREA_LABEL_EXISTS(image_0)) || \