bsp: Restore the ability to build firmware only

To allow us to continue to ship Corstone-1000 releases that only include
the firmware with the built in Linux image we need a way to build it
outside of the multiconfig builds.

Signed-off-by: Drew Reed <Drew.Reed@arm.com>

ci/corstone1000-firmware-only.yml

@@ -0,0 +1,25 @@
+---
+header:
+  version: 14
+
+local_conf_header:
+  rescuebuild: |
+    # Need to ensure the rescue linux options are selected
+    OVERRIDES .= ":firmware"
+
+    # Need to ensure we build with a small libc
+    TCLIBC="musl"
+
+  mass-storage: |
+    # Ensure the Mass Storage device is absent
+    FVP_CONFIG[board.msd_mmc.p_mmc_file] = "invalid.dat"
+
+  test-configuration: |
+    TEST_SUITES = "_qemutiny ping"
+    # Remove Dropbear SSH as it will not fit into the corstone1000 image.
+    IMAGE_FEATURES:remove = "ssh-server-dropbear"
+    CORE_IMAGE_EXTRA_INSTALL:remove = "ssh-pregen-hostkeys"
+
+target:
+  - corstone1000-flash-firmware-image
+  - perf