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>

arch/x86/cpu/i386/cpu.c

@@ -476,6 +476,8 @@ int x86_cpu_reinit_f(void)
 {
 	setup_identity();
 	setup_pci_ram_top();
+	if (locate_coreboot_table() >= 0)
+		gd->flags |= GD_FLG_SKIP_LL_INIT;
 
 	return 0;
 }