zephyr: migrate includes to <zephyr/...>

Zephyr has prefixed all of its includes with <zephyr/...>. While the
_old_ mode can still be used (CONFIG_LEGACY_INCLUDE_PATH) and is still
enabled by default, it's better to be prepared for its removal in the
future.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>
diff --git a/boot/zephyr/boot_serial_extensions.c b/boot/zephyr/boot_serial_extensions.c
index 763dedb..a026668 100644
--- a/boot/zephyr/boot_serial_extensions.c
+++ b/boot/zephyr/boot_serial_extensions.c
@@ -4,9 +4,9 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr.h>
-#include <drivers/flash.h>
-#include <mgmt/mcumgr/zephyr_groups.h>
+#include <zephyr/zephyr.h>
+#include <zephyr/drivers/flash.h>
+#include <zephyr/mgmt/mcumgr/zephyr_groups.h>
 
 #include <flash_map_backend/flash_map_backend.h>
 #include <sysflash/sysflash.h>