Merge branch 'ci' into 'master'

add ci

See merge request cats/mobile-transceiver-software!2

.gitlab-ci.yml

+image: "rust:latest"
+
+stages:
+  - lint
+  - build
+
+before_script:
+  - rustup target add thumbv7em-none-eabihf
+  - rustup component add rustfmt
+  - rustup component add clippy
+  - cargo install flip-link
+
+lint:
+  stage: lint
+  script:
+    - cargo fmt -- --check
+    - cargo clippy --all-features -- -D warnings
+
+build:
+  stage: build
+  script:
+    - cargo build --release
+  artifacts:
+    paths:
+      - target/thumbv7em-none-eabihf/release/cats-mobile-transceiver-mainboard