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>