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/Kconfig

@@ -47,5 +47,17 @@ config COMPRESSED_PAYLOAD_LZMA
 	  In order to reduce the size payloads take up in the ROM chip
 	  coreboot can compress them using the LZMA algorithm.
 
+config PAYLOAD_OPTIONS
+	string
+	default ""
+	help
+	  Additional cbfstool options for the payload
+
+config PAYLOAD_IS_FLAT_BINARY
+	def_bool n
+	help
+	  Add the payload to cbfs as a flat binary type instead of as an
+	  elf payload
+
 endmenu