Add toolchain definition for coreboot-sdk

Add a toolchain definition for coreboot-sdk, so that volteer can build
with coreboot-sdk instead by adding -t coreboot-sdk to the "zmake
configure".

BUG=b:175248887
TEST=zmake configure -b -t coreboot-sdk -B $(mktemp -d) \
         projects/experimental/volteer

Change-Id: I1ac9dbf96d893846077e9a7cdc970f57d7a4890d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/zephyr-chrome/+/2583409
Tested-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Jett Rink <jettrink@chromium.org>
Commit-Queue: Jack Rosenthal <jrosenth@chromium.org>
8 files changed