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>

Makefile

@@ -1260,6 +1260,7 @@ endif
 ifdef CONFIG_SPL_LOAD_FIT
 MKIMAGEFLAGS_u-boot.img = -f auto -A $(ARCH) -T firmware -C none -O u-boot \
 	-a $(CONFIG_SYS_TEXT_BASE) -e $(CONFIG_SYS_UBOOT_START) \
+	-p $(CONFIG_FIT_EXTERNAL_OFFSET) \
 	-n "U-Boot $(UBOOTRELEASE) for $(BOARD) board" -E \
 	$(patsubst %,-b arch/$(ARCH)/dts/%.dtb,$(subst ",,$(CONFIG_OF_LIST)))
 else