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/arch/x86/dts/Makefile b/arch/x86/dts/Makefile
index 431bbd8..ca2eab3 100644
--- a/arch/x86/dts/Makefile
+++ b/arch/x86/dts/Makefile
@@ -2,7 +2,8 @@
 	chromebox_panther.dtb \
 	crownbay.dtb \
 	galileo.dtb \
-	minnowmax.dtb
+	minnowmax.dtb \
+	qemu-x86.dtb
 
 targets += $(dtb-y)