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/x86_64/cpu.c b/arch/x86/cpu/x86_64/cpu.c
index 90925e4..4b64339 100644
--- a/arch/x86/cpu/x86_64/cpu.c
+++ b/arch/x86/cpu/x86_64/cpu.c
@@ -53,6 +53,7 @@
 	return 0;
 }
 
+#ifndef CONFIG_SYS_COREBOOT
 int checkcpu(void)
 {
 	return 0;
@@ -62,6 +63,7 @@
 {
 	return 0;
 }
+#endif
 
 int x86_cpu_reinit_f(void)
 {