Fix cache-url to use single '=' in test expression

When downloading the base image for the first time, the response
test expression fails with '[: 200: unexpected operator'.  This is
caused by using '==' in the test expression, which is not supported
in sh scripts.

Fixes bug 1195030

Change-Id: I66260814cb591371dc5c10f8436f90c2f18d78cf

elements/cache-url/bin/cache-url

@@ -35,10 +35,10 @@ else
 fi
 
 rcode=$(curl -o $tmp -w '%{http_code}' $url $time_cond)
-if [ "$rcode"  == "200" ] ; then
+if [ "$rcode" = "200" ] ; then
     echo $success
     mv $tmp $dest
-elif [ "$rcode" == "304" ] ; then
+elif [ "$rcode" = "304" ] ; then
     echo "Server copy has not changed. Using locally cached $url"
     rm -f $tmp
 else