ci: Fix environment variable check

The check for the variables were passing even tho they were not set,
resulting in bad syntax in the docker file:

 ENV DL_DIR
 ENV CCACHE_DIR '
 + docker build --network=host -t openpower/op-build-ubuntu1604 -
 Sending build context to Docker daemon   2.56kB
 Error response from daemon: ENV must have two arguments

I am no bash guru; this test is what is recommended by the first hit on
stackoverflow. Improve it if you know better.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/ci/build.sh b/ci/build.sh
index f663ef4..44852c6 100755
--- a/ci/build.sh
+++ b/ci/build.sh
@@ -105,10 +105,10 @@
 	    PROXY="RUN echo \"Acquire::http::Proxy \\"\"${http_proxy}/\\"\";\" > /etc/apt/apt.conf.d/000apt-cacher-ng-proxy"
 	  fi
         fi
-	if [[ -n "DL_DIR" ]]; then
+	if [ ! -z ${DL_DIR+x} ]; then
 	  DL_DIR_ENV="ENV DL_DIR $DL_DIR"
 	fi
-	if [[ -n "CCACHE_DIR" ]]; then
+	if [ ! -z ${CCACHE_DIR+x} ]; then
 	  CCACHE_DIR_ENV="ENV CCACHE_DIR $CCACHE_DIR"
 	fi