x86: Support QEMU x86 targets

This commit introduces the initial U-Boot support for QEMU x86 targets.
U-Boot can boot from coreboot as a payload, or directly without coreboot.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Merged in patch 'x86: qemu: Add CMD_NET to qemu-x86_defconfig
   https://patchwork.ozlabs.org/patch/479745/
diff --git a/configs/qemu-x86_defconfig b/configs/qemu-x86_defconfig
new file mode 100644
index 0000000..53cb553
--- /dev/null
+++ b/configs/qemu-x86_defconfig
@@ -0,0 +1,7 @@
+CONFIG_X86=y
+CONFIG_VENDOR_EMULATION=y
+CONFIG_TARGET_QEMU_X86=y
+CONFIG_CMD_NET=y
+CONFIG_OF_CONTROL=y
+CONFIG_OF_SEPARATE=y
+CONFIG_DEFAULT_DEVICE_TREE="qemu-x86"