Generate gdbstub-xml.c only when needed

First user of new config-devices.mak

Patchworks-ID: 35198
Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/configure b/configure
index 0cb4326..5c09c05 100755
--- a/configure
+++ b/configure
@@ -2302,8 +2302,8 @@
   for x in $gdb_xml_files; do
     list="$list $source_path/gdb-xml/$x"
   done
+  echo "TARGET_XML_FILES=$list" >> $config_target_mak
 fi
-echo "TARGET_XML_FILES=$list" >> $config_target_mak
 
 case "$target_arch2" in
   arm|armeb|m68k|microblaze|mips|mipsel|mipsn32|mipsn32el|mips64|mips64el|ppc|ppc64|ppc64abi32|ppcemb|sparc|sparc64|sparc32plus)