download/u-boot: Change to download target before running extra.sh

The U-Boot download script does its work from the repository root
instead going into the newly created dirs, unlike the coreboot
counterpart. It should run the board-specific extra.sh files with the
downloaded paths as their working directory. Do so by a subshell.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>

resources/scripts/download/u-boot

@@ -181,7 +181,7 @@ downloadfor() {
 	# extra.sh on a per-board basis
 	# In fact, extra.sh can be used for anything you want.
 	if [ -f "resources/u-boot/${board}/extra.sh" ]; then
-		"resources/u-boot/${board}/extra.sh" || touch build_error
+		( cd "${ubtree}" && "../../resources/u-boot/${board}/extra.sh"; ) || touch build_error
 		if [ -f build_error ]; then
 			return 1
 		fi