x86: Allow building an SPL image for coreboot

Coreboot runs in 32-bit mode and cannot run a 64-bit U-Boot. To get around
this we can build a combined image with 32-bit SPL and 64-bit U-Boot. Add
a build rule and binman definition for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/Makefile b/Makefile
index 6bb9cf5..cc99873 100644
--- a/Makefile
+++ b/Makefile
@@ -926,6 +926,9 @@
 ifneq ($(BUILD_ROM)$(CONFIG_BUILD_ROM),)
 ALL-$(CONFIG_X86_RESET_VECTOR) += u-boot.rom
 endif
+ifeq ($(CONFIG_SYS_COREBOOT)$(CONFIG_SPL),yy)
+ALL-$(CONFIG_BINMAN) += u-boot-x86-with-spl.bin
+endif
 
 # Build a combined spl + u-boot image for sunxi
 ifeq ($(CONFIG_ARCH_SUNXI)$(CONFIG_SPL),yy)
@@ -1626,6 +1629,9 @@
 endif
 endif
 
+u-boot-x86-with-spl.bin: spl/u-boot-spl.bin u-boot.bin FORCE
+	$(call if_changed,binman)
+
 ifneq ($(CONFIG_TEGRA),)
 ifneq ($(CONFIG_BINMAN),)
 # Makes u-boot-dtb-tegra.bin u-boot-tegra.bin u-boot-nodtb-tegra.bin