zedboard: Generate BOOT.BIN automatically

The U-Boot v2015.07 builds working SPL without a manual
intervention. The BOOT.BIN is generated from the standard
SPL so no BR2_TARGET_UBOOT_SPL_NAME redefinition is needed.
The SPL requires the u-boot-dtb.img file to boot properly.

Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>