Makefile: Add target for building bootable SPL image for SoCFPGA

Add build target for generating boot partition images recognised by
the SoCFPGA BootROM. The SoCFPGA BootROM expects four copies of the
u-boot-spl-dtb.sfp at the beginning of boot partition. Those are
u-boot-spl-dtb.bin augmented by a header with which the BootROM can
work. The u-boot-dtb.img uImage is appended to this to produce a
full boot partition image, the u-boot-with-spl-dtb.sfp . This is
the name of the final target.

Signed-off-by: Marek Vasut <marex@denx.de>