ARM: rmobile: Build u-boot-elf.srec on Gen3

The u-boot-elf.srec is needed for the minimon update on Gen3 SoCs,
conveniently generate this file.

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

include/configs/rcar-gen3-common.h

@@ -13,6 +13,7 @@
 #include <asm/arch/rmobile.h>
 
 #define CONFIG_REMAKE_ELF
+#define CONFIG_BUILD_TARGET	"u-boot-elf.srec"
 
 /* boot option */