x86: coreboot: Enable standard boot

Enable bootstd options and provide instructions on how to boot a linux
distro using coreboot.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/configs/coreboot_defconfig b/configs/coreboot_defconfig
index aedad4c..311ca66 100644
--- a/configs/coreboot_defconfig
+++ b/configs/coreboot_defconfig
@@ -10,6 +10,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_BOOTSTD_FULL=y
+CONFIG_BOOTSTD_DEFAULTS=y
 CONFIG_SYS_MONITOR_BASE=0x01110000
 CONFIG_SHOW_BOOT_PROGRESS=y
 CONFIG_USE_BOOTARGS=y