zephyr: cleanup flash driver name

The flash driver name was changed upstream during the v1.11
development cycle. Remove a shim which kept the old name around in the
Zephyr port now that v1.11 is out.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index 1f80dac..6dda3bd 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -99,9 +99,9 @@
 
     os_heap_init();
 
-    boot_flash_device = device_get_binding(FLASH_DRIVER_NAME);
+    boot_flash_device = device_get_binding(FLASH_DEV_NAME);
     if (!boot_flash_device) {
-        BOOT_LOG_ERR("Flash device not found");
+        BOOT_LOG_ERR("Flash device %s not found", FLASH_DEV_NAME);
         while (1)
             ;
     }