Makefile: add u-boot-elf.srec target

The u-boot-elf.srec is needed for some platforms, add target to generate this file.
Signed-off-by: NMasaru Nagai <masaru.nagai.vx@renesas.com>
Signed-off-by: NHiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: NMarek Vasut <marek.vasut+renesas@gmail.com>
Cc: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

Makefile

@@ -926,6 +926,11 @@ OBJCOPYFLAGS_u-boot.srec := -O srec
 u-boot.hex u-boot.srec: u-boot FORCE
 	$(call if_changed,objcopy)
 
+OBJCOPYFLAGS_u-boot-elf.srec := $(OBJCOPYFLAGS_u-boot.srec)
+
+u-boot-elf.srec: u-boot.elf FORCE
+	$(call if_changed,objcopy)
+
 OBJCOPYFLAGS_u-boot-nodtb.bin := -O binary \
 		$(if $(CONFIG_X86_16BIT_INIT),-R .start16 -R .resetvec)