uboot-imx

spl: Use u-boot.img instead of u-boot.bin

In SD boot, the magic number of u-boot image will be checked.
For LS102xA, u-boot.bin doesn't have the magic number. So use
u-boot.img which includes the magic number instead of u-boot.bin
when producing u-boot-with-spl-pbl.bin.

Signed-off-by: Alison Wang <alison.wang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>