Makefile: add u-boot-spl.srec target

The u-boot-spl.srec is needed for some platforms, add target to generate this file.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Tom Rini <trini@konsulko.com>

Makefile

@@ -931,6 +931,11 @@ OBJCOPYFLAGS_u-boot-elf.srec := $(OBJCOPYFLAGS_u-boot.srec)
 u-boot-elf.srec: u-boot.elf FORCE
 	$(call if_changed,objcopy)
 
+OBJCOPYFLAGS_u-boot-spl.srec = $(OBJCOPYFLAGS_u-boot.srec)
+
+spl/u-boot-spl.srec: spl/u-boot-spl FORCE
+	$(call if_changed,objcopy)
+
 OBJCOPYFLAGS_u-boot-nodtb.bin := -O binary \
 		$(if $(CONFIG_X86_16BIT_INIT),-R .start16 -R .resetvec)