automation/cirrus-ci: use matrix keyword to generate per-version build tasks

Move the current logic to use the matrix keyword to generate a task for
each version of FreeBSD we want to build Xen on.  The matrix keyword
however cannot be used in YAML aliases, so it needs to be explicitly used
inside of each task, which creates a bit of duplication.  At least abstract
the FreeBSD minor version numbers to avoid repetition of image names.

Note that the full build uses matrix over an env variable instead of using
it directly in image_family.  This is so that the alias can also be set
based on the FreeBSD version, in preparation for adding further tasks that
will depend on the full build having finished.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

.cirrus.yml

@@ -1,13 +1,9 @@
 # https://cirrus-ci.org/guide/tips-and-tricks/#sharing-configuration-between-tasks
-freebsd_13: &FREEBSD_13
-  freebsd_instance:
-    image_family: freebsd-13-5
-freebsd_14: &FREEBSD_14
-  freebsd_instance:
-    image_family: freebsd-14-2
-freebsd_15: &FREEBSD_15
-  freebsd_instance:
-    image_family: freebsd-15-0-snap
+freebsd_versions: &FREEBSD_VERSIONS
+  env:
+    FREEBSD_LEGACY:     freebsd-13-5
+    FREEBSD_PRODUCTION: freebsd-14-2
+    FREEBSD_CURRENT:    freebsd-15-0-snap
 
 freebsd_template: &FREEBSD_ENV
   environment:
@@ -24,7 +20,24 @@ freebsd_configure_artifacts: &FREEBSD_CONFIGURE_ARTIFACTS
       path: xen-config
       type: text/plain
 
-freebsd_full_build_template: &FREEBSD_FULL_BUILD_TEMPLATE
+task:
+  name: 'FreeBSD: full build'
+
+  # It's not possible to use the matrix keyword in YAML aliases, as they
+  # keyword usage is restricted to task, docker_builder or pipe.  Attempting to
+  # use a YAML alias with the duplicated keys doesn't work either. Use an env
+  # variable so the version can also be appended to the task alias.
+  << : *FREEBSD_VERSIONS
+  env:
+    matrix:
+      FREEBSD_VERSION: $FREEBSD_LEGACY
+      FREEBSD_VERSION: $FREEBSD_PRODUCTION
+      FREEBSD_VERSION: $FREEBSD_CURRENT
+
+  alias: freebsd_full_$FREEBSD_VERSION
+  freebsd_instance:
+    image_family: $FREEBSD_VERSION
+
   << : *FREEBSD_ENV
 
   install_script: pkg install -y seabios gmake ninja bash
@@ -42,7 +55,20 @@ freebsd_full_build_template: &FREEBSD_FULL_BUILD_TEMPLATE
   build_script:
     - gmake -j`sysctl -n hw.ncpu` clang=y
 
-freebsd_randconfig_template: &FREEBSD_RANDCONFIG_TEMPLATE
+task:
+  name: 'FreeBSD: randconfig build'
+
+  # It's not possible to use the matrix keyword in YAML aliases, as they
+  # keyword usage is restricted to task, docker_builder or pipe.  Attempting to
+  # use a YAML alias with the duplicated `image_family` keys doesn't work
+  # either. Abstract the version numbers at least.
+  << : *FREEBSD_VERSIONS
+  freebsd_instance:
+    matrix:
+      image_family: $FREEBSD_LEGACY
+      image_family: $FREEBSD_PRODUCTION
+      image_family: $FREEBSD_CURRENT
+
   << : *FREEBSD_ENV
 
   install_script: pkg install -y gmake python3 bison
@@ -56,33 +82,3 @@ freebsd_randconfig_template: &FREEBSD_RANDCONFIG_TEMPLATE
 
   build_script:
     - gmake -j`sysctl -n hw.ncpu` build-xen clang=y
-
-task:
-  name: 'FreeBSD 13: full build'
-  << : *FREEBSD_13
-  << : *FREEBSD_FULL_BUILD_TEMPLATE
-
-task:
-  name: 'FreeBSD 14: full build'
-  << : *FREEBSD_14
-  << : *FREEBSD_FULL_BUILD_TEMPLATE
-
-task:
-  name: 'FreeBSD 15: full build'
-  << : *FREEBSD_15
-  << : *FREEBSD_FULL_BUILD_TEMPLATE
-
-task:
-  name: 'FreeBSD 13: randconfig'
-  << : *FREEBSD_13
-  << : *FREEBSD_RANDCONFIG_TEMPLATE
-
-task:
-  name: 'FreeBSD 14: randconfig'
-  << : *FREEBSD_14
-  << : *FREEBSD_RANDCONFIG_TEMPLATE
-
-task:
-  name: 'FreeBSD 15: randconfig'
-  << : *FREEBSD_15
-  << : *FREEBSD_RANDCONFIG_TEMPLATE