x86: coreboot: Allow building an SPL image

Make a few adjustments to allow us to build an SPL image for coreboot.

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/arch/x86/cpu/intel_common/Makefile b/arch/x86/cpu/intel_common/Makefile
index 1736bd2..374803b 100644
--- a/arch/x86/cpu/intel_common/Makefile
+++ b/arch/x86/cpu/intel_common/Makefile
@@ -32,6 +32,8 @@
 
 ifdef CONFIG_SPL
 ifndef CONFIG_SPL_BUILD
+ifndef CONFIG_SYS_COREBOOT
 obj-y += cpu_from_spl.o
 endif
 endif
+endif