kbuild: fix warning when domainname is not available

Otherwise we get:
"dnsdomainname: Unknown host"
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/mkcompile_h

@@ -66,9 +66,13 @@ UTS_TRUNCATE="sed -e s/\(.\{1,$UTS_LEN\}\).*/\1/"
   echo \#define LINUX_COMPILE_HOST \"`hostname | $UTS_TRUNCATE`\"
 
   if [ -x /bin/dnsdomainname ]; then
-    echo \#define LINUX_COMPILE_DOMAIN \"`dnsdomainname | $UTS_TRUNCATE`\"
+    domain=`dnsdomainname 2> /dev/null`
   elif [ -x /bin/domainname ]; then
-    echo \#define LINUX_COMPILE_DOMAIN \"`domainname | $UTS_TRUNCATE`\"
+    domain=`domainname 2> /dev/null`
+  fi
+
+  if [ -n "$domain" ]; then
+    echo \#define LINUX_COMPILE_DOMAIN \"`echo $domain | $UTS_TRUNCATE`\"
   else
     echo \#define LINUX_COMPILE_DOMAIN
   fi