Payloads: Add U-Boot as a coreboot-payload

- Add Kconfig and Makefile options to use U-Boot as a payload.
- Add Kconfig option for extra cbfstool command line arguments.
- Add Kconfig & Makefile option to load the payload as a flat binary.
- Add u-boot directory to .gitignore.

This is currently working for X-86 only.

Graphics worked in U-Boot correctly by initializing the VBIOS and
setting up a console mode.

Tested in QEMU and on Minnowboard Max.  Got into U-Boot, have not
booted an OS yet.

Change-Id: Ia122a4ad7cd7d96107c1552b0376c8106ca8fb92
Signed-off-by: Martin Roth <martinroth@google.com>
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/12714
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

payloads/external/U-Boot/Kconfig.name

@@ -0,0 +1,11 @@
+config PAYLOAD_UBOOT
+	bool "U-Boot (Experimental)"
+	depends on ARCH_X86
+	help
+	  Select this option if you want to build a coreboot image
+	  with a U-Boot payload.
+
+	  See http://coreboot.org/Payloads and U-Boot's documentation
+	  at http://git.denx.de/?p=u-boot.git;a=blob;f=doc/README.x86
+	  for more information.
+