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>
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index 4879d0a..239ee48 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -163,6 +163,10 @@
 ALL-y	+= $(obj)/$(BOARD)-spl.bin
 endif
 
+ifdef CONFIG_ARCH_SOCFPGA
+ALL-y	+= $(obj)/$(SPL_BIN)-dtb.sfp
+endif
+
 ifdef CONFIG_SUNXI
 ALL-y	+= $(obj)/sunxi-spl.bin
 endif
@@ -231,6 +235,12 @@
 LDFLAGS_$(SPL_BIN) += -Ttext $(CONFIG_SPL_TEXT_BASE)
 endif
 
+ifdef CONFIG_ARCH_SOCFPGA
+MKIMAGEFLAGS_$(SPL_BIN)-dtb.sfp = -T socfpgaimage
+$(obj)/$(SPL_BIN)-dtb.sfp: $(obj)/$(SPL_BIN)-dtb.bin FORCE
+	$(call if_changed,mkimage)
+endif
+
 ifdef CONFIG_SUNXI
 quiet_cmd_mksunxiboot = MKSUNXI $@
 cmd_mksunxiboot = $(objtree)/tools/mksunxiboot $< $@