add minimal CI script for GitLab

.gitlab-ci.yml

+image: debian:buster
+
+stages:
+  - test
+
+test:
+  stage: test
+  script:
+  - apt-get update -qy
+  - apt-get install -y build-essential
+  - ./mp arch=posix app=ledblink loop=1 timer_s=1
+  - ./mp arch=posix app=sysinfo
+  - ./mpm arch=posix app=donothing