Add extremely basic CI

It compiles :)

.gitlab-ci.yml

+image: "debian"
+
+build:
+    stage: build
+    before_script:
+        - apt update -qq && apt install -y -qq gcc-arm-none-eabi python3-pip git
+        - pip3 install crc16
+        - ln -s /usr/bin/python3 /usr/bin/python
+    script:
+        - git clone https://git.card10.badge.events.ccc.de/card10/firmware.git
+        - make -C ports/card10 SDK_LOCATION=`pwd`/firmware/sdk