Update drone.yml

drone.yml

     - apt-get update
     - apt-get --yes upgrade
     - apt-get --yes install build-essential libc6-dev libhidapi-dev libssl-dev
-    - export MAKEFLAGS=$MAKEFLAGS 
+    - export MAKEFLAGS
     - cd src && make 
     - cd contrib && make 
     - cd .. ; ls -l radangel contrib/websockify ; md5sum radangel contrib/websockify ; cd ..