zephyr: remove flash_device_get_binding

remove flash_device_get_binding as we can utilizes DEVICE_DT_GET to
get the flash_dev pointer set at build time.  This removes usage of
device_get_binding and handles the lack of 'label' properties in
the devicetree better.

Signed-off-by: Kumar Gala <galak@kernel.org>
diff --git a/boot/zephyr/include/target.h b/boot/zephyr/include/target.h
index 5562434..77a4d74 100644
--- a/boot/zephyr/include/target.h
+++ b/boot/zephyr/include/target.h
@@ -27,15 +27,11 @@
 
 #endif /* !defined(MCUBOOT_TARGET_CONFIG) */
 
-#if DT_NODE_HAS_PROP(DT_INST(0, jedec_spi_nor), label)
-#define JEDEC_SPI_NOR_0_LABEL DT_LABEL(DT_INST(0, jedec_spi_nor))
-#endif
-
 /*
  * Sanity check the target support.
  */
 #if (!defined(CONFIG_XTENSA) && !DT_HAS_CHOSEN(zephyr_flash_controller)) || \
-    (defined(CONFIG_XTENSA) && !defined(JEDEC_SPI_NOR_0_LABEL)) || \
+    (defined(CONFIG_XTENSA) && !DT_NODE_EXISTS(DT_INST(0, jedec_spi_nor))) || \
     !defined(FLASH_ALIGN) ||                  \
     !(FLASH_AREA_LABEL_EXISTS(image_0)) || \
     !(FLASH_AREA_LABEL_EXISTS(image_1) || CONFIG_SINGLE_APPLICATION_SLOT) || \