zephyr: use <zephyr/kernel.h>

<zephyr/zephyr.h> is just a shim to the <zephyr/kernel.h> header, better
use it directly as it is explicit that Kernel APIs are being included.

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 a026668..c2f1b93 100644
--- a/boot/zephyr/boot_serial_extensions.c
+++ b/boot/zephyr/boot_serial_extensions.c
@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include <zephyr/drivers/flash.h>
 #include <zephyr/mgmt/mcumgr/zephyr_groups.h>
 
diff --git a/boot/zephyr/flash_map_extended.c b/boot/zephyr/flash_map_extended.c
index c902978..d9e39c1 100644
--- a/boot/zephyr/flash_map_extended.c
+++ b/boot/zephyr/flash_map_extended.c
@@ -5,7 +5,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include <zephyr/devicetree.h>
 #include <zephyr/drivers/flash.h>
 
diff --git a/boot/zephyr/hooks_sample.c b/boot/zephyr/hooks_sample.c
index b151d0d..baa2b75 100644
--- a/boot/zephyr/hooks_sample.c
+++ b/boot/zephyr/hooks_sample.c
@@ -15,7 +15,7 @@
  */
 
 #include <assert.h>
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include "bootutil/image.h"
 #include "bootutil/bootutil.h"
 #include "bootutil/fault_injection_hardening.h"
diff --git a/boot/zephyr/main.c b/boot/zephyr/main.c
index b1c394f..6cf1a59 100644
--- a/boot/zephyr/main.c
+++ b/boot/zephyr/main.c
@@ -17,7 +17,7 @@
  */
 
 #include <assert.h>
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include <zephyr/devicetree.h>
 #include <zephyr/drivers/gpio.h>
 #include <zephyr/sys/__assert.h>
diff --git a/boot/zephyr/os.c b/boot/zephyr/os.c
index 8588a91..048cb2b 100644
--- a/boot/zephyr/os.c
+++ b/boot/zephyr/os.c
@@ -17,7 +17,7 @@
  * under the License.
  */
 
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include <string.h>
 
 #include "os/os_heap.h"
diff --git a/boot/zephyr/serial_adapter.c b/boot/zephyr/serial_adapter.c
index 4934f6d..309d72a 100644
--- a/boot/zephyr/serial_adapter.c
+++ b/boot/zephyr/serial_adapter.c
@@ -18,7 +18,7 @@
 #include <zephyr/drivers/uart.h>
 #include <assert.h>
 #include <string.h>
-#include <zephyr/zephyr.h>
+#include <zephyr/kernel.h>
 #include "bootutil/bootutil_log.h"
 #include <zephyr/usb/usb_device.h>