u-boot-update: Fix location of fdt dir

U_BOOT_FDT_DIR with newer u-boot-menu is meant to be the path inside the partition which has the dtbs; Adjust our default for U_BOOT_FDT_DIR to match that to ensure the fdtdir gets generated properly.

Signed-off-by: Sjoerd Simons sjoerd.simons@collabora.co.uk