You are obvisouly using an old Makefile in imx-bootlets-src-10.12.01
As many of the commented out lines in mine or not in yours, and you are missing the elftosb instruction to generate the uboot boot stream.
Please modify your Makefile (in imx-bootlets-src-):
This is what you execute to generate the mx28_ivt_linux.sb ->
ifeq "$(DFT_IMAGE)" "$(wildcard $(DFT_IMAGE))"
@echo "by using the rootfs/boot/zImage"
sed -i 's,[^ *]zImage.*;,\tzImage="$(DFT_IMAGE)";,' linux.bd
sed -i 's,[^ *]zImage.*;,\tzImage="$(DFT_IMAGE)";,' linux_ivt.bd
elftosb -z -c ./linux.bd -o i$(ARCH)_linux.sb
elftosb -z -f imx28 -c ./linux_ivt.bd -o i$(ARCH)_ivt_linux.sb
-> this part is obviously not executed as the text below is not being displayed:
@echo "by using the rootfs/boot/u-boot"
sed -i 's,[^ *]u_boot.*;,\tu_boot="$(DFT_UBOOT)";,' uboot.bd
sed -i 's,[^ *]u_boot.*;,\tu_boot="$(DFT_UBOOT)";,' uboot_ivt.bd
elftosb -z -c ./uboot.bd -o i$(ARCH)_uboot.sb
elftosb -z -f imx28 -c ./uboot_ivt.bd -o i$(ARCH)_ivt_uboot.sb
Therefore add these above lines in the first branch of the ifeq.