download/coreboot: use global variables

i'm going to move the config recursion check
into a separate function, and global variables
make it easier to handle

resources/scripts/download/coreboot

@@ -26,6 +26,10 @@
 [ "x${DEBUG+set}" = 'xset' ] && set -v
 set -u -e
 
+_board=""
+cbtree=""
+cbrevision=""
+
 main()
 {
 	if [ $# -eq 1 ] && [ "$1" = "--help" ] ; then