gitlab/azure: x86: Add a coreboot test

Coreboot supports U-Boot as a payload and this recently got a bit of a
facelist. Add a test for this.

For now this uses a binary build of coreboot (v4.15). Future work could
potentially build it from source, but we need to figure out the
toolchain problems first, since coreboot uses its own toolchain. It
turns out that this is tricky, because coreboot fails to build with a
vanilla gcc.

This needs some changes to the hooks scripts as well. An example build
is at https://source.denx.de/u-boot/custodians/u-boot-dm/-/jobs/359687



Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>