.gitlab-ci.yml: re-use build.sh to avoid maintaining the list of necessary...

.gitlab-ci.yml: re-use build.sh to avoid maintaining the list of necessary environment variables in two places

.gitlab-ci.yml

@@ -5,7 +5,7 @@ build:
     - apt-get update
     - apt-get --yes install --no-install-recommends gcc-aarch64-linux-gnu build-essential bison flex device-tree-compiler
     - cp mntreform-config .config
-    - env CROSS_COMPILE=aarch64-linux-gnu- ARCH=arm make flash.bin
+    - ./build.sh
   artifacts:
     paths:
       - "flash.bin"