am335x: Build an omap style header and an image with it at the front.

Loading on an OMAP SOC requires that the first sector of the image have a
configuration header, and, when not an execute in place image, an additional
header which describes how big the image is and where it should be loaded.
This change adds some infrastructure to statically build that header using C
code, and to paste the header onto the front of coreboot.rom in a new top
level target file called MLO.

The configuration header we're using is as inert as possible, in line with
what U-Boot is doing. I think it could be used to give additional
configuration parameters to the built-in ROM on the SOC, but we don't need to
do that, and there didn't seem to be any actual documentation how to do that.
Because the header is built from C and is defined per CPU, it would be
possible to include extra settings in other CPUs if desired.

Adding a new top level build target is a bit disruptive, but should be
contained to the am335x directory and not interfere with other mainboards.

Change-Id: I06d346a4050c20963b3c7c6e8a152070bf2d145a
Signed-off-by: Gabe Black <gabeblack@chromium.org>
Reviewed-on: http://review.coreboot.org/3332
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

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