spl, Makefile: Take external offset into account for fit images

When building a FIT image for SPL to load also take into account the
configured external offset. This allows the SPL to load the u-boot FIT
image correctly when configuring SECURE_BOOT on i.mx6 is enabled
(with CONFIG_FIT_EXTERNAL_OFFSET=0x3000).

Note this doesn't allow secure booting; but it does allow an unsigned
SPL/u-boot with secure boot support eanbled to boot on open device.

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