[PATCH] BaseTools: Add COREBOOT tools definition

[Patrick Georgi <pgeorgi@google.com>]

That toolset is defined as the current coreboot toolchain[0] at commit
time. Right now this is gcc 6.3, iasl and GNU make, with minor patches
as deemed necessary for firmware development by the coreboot maintainers.

COREBOOT is primarily supposed to build CorebootPayloadPkg (and its
dependencies), but should be suitable for other builds as well.
It's tested to build a usable Ovmf image.

For now, COREBOOT is a copy of the GCC5 rules, but it may diverge over
time.

[0] As built by https://review.coreboot.org/cgit/coreboot.git/tree/util/
crossgcc

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Patrick Georgi <pgeorgi@google.com>

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