download/coreboot: run extra.sh from cbtree

to my knowledge, this feature has never been used,
but lbmk permits resources/coreboot/boardname/extra.sh
to execute, as provided by the maintainer, with working
directory set to: coreboot/boardname

this could be used to extend lbmk in a number of ways

for example, it could be used to patch 3rdparty/

it could also be used to break coreboot in creative
and novel ways. hint hint.

resources/scripts/download/coreboot

@@ -188,8 +188,8 @@ prepare_new_coreboot_tree()
 
 	# extra.sh can be used for anything
 	# but should *only* be a last resort
-	if [ -f "../../resources/coreboot/${_board}/extra.sh" ]; then
-		"../../resources/coreboot/${_board}/extra.sh" || \
+	if [ -f "../../resources/coreboot/${cbtree}/extra.sh" ]; then
+		"../../resources/coreboot/${cbtree}/extra.sh" || \
 			exit 1
 	fi
 	)