boards: nxp: mr_canhubk3: add mcuboot variant and document sysbuild

Add an mcuboot board variant under the S32K344 SoC in board.yml.
New target is - mr_canhubk3/s32k344/mcuboot. This enables a
single sysbuild invocation to build MCUboot and the application.
Also make a common dtsi for all mr_canhubk3 board variants.

Extend the board documentation with an “MCUboot (sysbuild)” section
that:

Shows one-command sysbuild to produce both MCUboot and app
Documents signing via sysbuild (SB_CONFIG_BOOT_SIGNATURE_*),
and where the signed artifacts are generated.

Provides simple flashing instructions and a troubleshooting
note about using a 1 KiB (0x400) image header and not emitting
an IVT for chain-loaded apps.

Signed-off-by: Sumit Batra <sumit.batra@nxp.com>

Author: sumitbatra-nxp

boards/nxp/mr_canhubk3/Kconfig.sysbuild

@@ -0,0 +1,20 @@
+# Copyright 2025 NXP
+# SPDX-License-Identifier: Apache-2.0
+
+
+if BOARD_MR_CANHUBK3_S32K344_MCUBOOT
+
+choice BOOTLOADER
+	default BOOTLOADER_MCUBOOT
+endchoice
+
+# Use direct-XIP mode (dual-slot) on S32K3
+choice MCUBOOT_MODE
+	default MCUBOOT_MODE_DIRECT_XIP
+endchoice
+
+choice BOOT_SIGNATURE_TYPE
+	default BOOT_SIGNATURE_TYPE_RSA
+endchoice
+
+endif # BOARD_MR_CANHUBK3_S32K344_MCUBOOT

boards/nxp/mr_canhubk3/board.yml

@@ -4,3 +4,5 @@ board:
   vendor: nxp
   socs:
     - name: s32k344
+      variants:
+        - name: mcuboot

boards/nxp/mr_canhubk3/doc/index.rst

@@ -296,6 +296,37 @@ For example, to erase and verify flash content:
 
    west flash -r trace32 --startup-args elfFile=build/zephyr/zephyr.elf loadTo=flash eraseFlash=yes verifyFlash=yes
 
+MCUboot
+=======
+
+This board supports app chain-loading using MCUboot.
+
+Build & Flash
+-------------
+
+To build MCUboot and the ``flash_shell`` sample application together and
+generate HEX files suitable for flashing, run:
+
+.. code-block:: console
+
+   west build -p -b mr_canhubk3/s32k344/mcuboot samples/drivers/flash_shell --sysbuild
+   west flash
+
+The resulting artifacts are:
+
+* MCUboot: ``build/mcuboot/zephyr/zephyr.hex``
+* App (unsigned): ``build/flash_shell/zephyr/zephyr.hex``
+
+Troubleshooting
+---------------
+
+    If MCUboot prints “Image in the primary slot is not valid” or stalls after
+    “Jumping to the first image slot”, the app was likely signed with a 512-byte header.
+    Re-sign with --header-size 0x400 and re-flash.
+
+    Do not add an IVT to MCUboot-chainloaded applications;
+    it’s only emitted for standalone/XIP images or MCUboot itself.
+
 Debugging
 =========