gitlab ci: fix posix target

.gitlab-ci.yml

@@ -18,6 +18,7 @@ build_posix:
   stage: build
   script:
   - mkdir -p build
+  - touch .config
   - ./mp arch=posix app=donothing
   - ./mp arch=posix app=ledblink loop=1 timer_s=1
   - ./mp arch=posix app=sysinfo