arm-bsp/docs,kas:corstone1000: Add SSH image build support

Add a shared Corstone-1000 kas configuration for building SSH-enabled
mass storage images across the supported Corstone-1000 machines.

Enable Dropbear SSH in the mass storage OS image, add pregenerated SSH
host keys for FVP builds, and document the SSH image build flow in the
user guides.

SSH support is too large for the flash OS image, so provide a dedicated
`core-image-minimal` kas configuration for SSH-enabled mass storage
images instead.

Signed-off-by: Alex Chapman <alex.chapman@arm.com>
Signed-off-by: Hugues KAMBA MPIANA <hugues.kambampiana@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

kas/corstone1000-ssh.yml

@@ -0,0 +1,13 @@
+header:
+  version: 14
+  includes:
+    - ci/debug.yml
+
+local_conf_header:
+  sshd: |
+    IMAGE_FEATURES += "ssh-server-dropbear"
+  fvp_sshkeys: |
+    CORE_IMAGE_EXTRA_INSTALL:append = " ssh-pregen-hostkeys"
+
+target:
+  - core-image-minimal

meta-arm-bsp/documentation/corstone1000/user-guide.rst

@@ -290,6 +290,28 @@ The output binaries run in the Corstone-1000 platform are the following:
  - The External System Processor firmware: ``${WORKSPACE}/build/tmp/deploy/images/corstone1000-${TARGET}/es_flashfw.bin``
  - The internal firmware flash image: ``${WORKSPACE}/build/tmp/deploy/images/corstone1000-${TARGET}/corstone1000-flash-firmware-image-corstone1000-${TARGET}.wic``
 
+Build with SSH
+--------------
+
+The ``meta-arm/kas/corstone1000-${TARGET}.yml`` build produces an image for
+booting from flash.
+
+To build a bootable mass storage OS image with Dropbear SSH enabled, run:
+
+.. code-block:: console
+
+    kas build meta-arm/ci/corstone1000-${TARGET}.yml:meta-arm/kas/corstone1000-ssh.yml
+
+The mass storage OS image can be found at ``${WORKSPACE}/build/tmp/deploy/images/corstone1000-${TARGET}/core-image-minimal-corstone1000-${TARGET}.wic``
+
+.. note::
+
+    For the FVP, the generated ``core-image-minimal-corstone1000-fvp.fvpconf``
+    configures the mass storage OS image using ``board.msd_mmc.p_mmc_file``.
+
+    For the MPS3 platform, write the ``*.wic`` image directly to the mass storage device.
+
+
 .. _flashing-firmware-images:
 
 Flash