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