uboot: Deploy default symlinks with fitImage

Some image recipes uses ${DEPLOY_DIR_IMAGE}/${UBOOT_BINARY} to create
their images. Force the re-creation of those symlinks pointing to the
u-boot-fitImage in case UBOOT_FITIMAGE_ENABLE is set.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>