doc/../dfu/bootloader_keys: mark ed25519 as only supported

Specifies:
ed25519 signature type is only supported signature on nRF54l SoC's
rest are for evaluation

Signed-off-by: Andrzej Puzdrowski <[email protected]>

Author: nvlsianpu

doc/nrf-bm/app_dev/dfu/bootloader_keys.rst

@@ -11,12 +11,17 @@ For testing and production use cases, unique signing keys must be generated and
 Signature type
 **************
 
-MCUboot in |BMshort| supports the following signature types:
+MCUboot in |BMshort| allow a few signatures types.
+The ed25519 signature type is recommended as supported for nRF54L Series devices with cryptographic hardware support (CRACEN and KMU).
+It is recommended to use pure version of ed25519 signature (to enabled use :kconfig:option:`SB_CONFIG_BM_BOOT_IMG_HASH_ALG_PURE``).
+Rest of the signature types are for evaluation purpose only and are inherited from the MCUboot project.
+
+The available signature types are listed in the table below:
 
 +------------+----------------------------------------------------------------------+-----------------------------------------------------------------------------+
 | Type       | Description                                                          | Sysbuild Kconfig                                                            |
 +============+======================================================================+=============================================================================+
-| None       | No signature verification (insecure)                                 | :kconfig:option:`SB_CONFIG_BM_BOOTLOADER_MCUBOOT_SIGNATURE_TYPE_NONE`       |
+| None       | No signature verification (insecure, development helper)             | :kconfig:option:`SB_CONFIG_BM_BOOTLOADER_MCUBOOT_SIGNATURE_TYPE_NONE`       |
 +------------+----------------------------------------------------------------------+-----------------------------------------------------------------------------+
 | RSA        | RSA-2048 or RSA-3072 signature                                       | :kconfig:option:`SB_CONFIG_BM_BOOTLOADER_MCUBOOT_SIGNATURE_TYPE_RSA`        |
 +------------+----------------------------------------------------------------------+-----------------------------------------------------------------------------+