Add Cirrus CI config file to support CI builds (in downstream projects)

Make use of the CI smoke test script added in r346080.

Reviewed by:	imp, lwhsu
Sponsored by:	The FreeBSD Foundation

.cirrus.yml

@@ -0,0 +1,19 @@
+# $FreeBSD$
+
+freebsd_instance:
+  image: freebsd-12-0-release-amd64
+  cpu: 8
+  memory: 24G
+
+env:
+  CIRRUS_CLONE_DEPTH: 1
+
+task:
+  timeout_in: 90m
+  install_script:
+  - pkg install -y qemu-devel
+  - fetch https://people.freebsd.org/~emaste/OVMF.fd
+  script:
+  - make -j$(sysctl -n hw.ncpu) WITHOUT_TOOLCHAIN=yes buildworld buildkernel
+  test_script:
+  - sh tools/boot/ci-qemu-test.sh