x86: Add a way to detect running from coreboot

If U-Boot is running from coreboot we need to skip low-level init. Add
an way to detect this and to set the gd flag.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/i386/cpu.c b/arch/x86/cpu/i386/cpu.c
index cc20456..0312a26 100644
--- a/arch/x86/cpu/i386/cpu.c
+++ b/arch/x86/cpu/i386/cpu.c
@@ -476,6 +476,8 @@
 {
 	setup_identity();
 	setup_pci_ram_top();
+	if (locate_coreboot_table() >= 0)
+		gd->flags |= GD_FLG_SKIP_LL_INIT;
 
 	return 0;
 }