xcompile: Add CROSS_COMPILE_* variable

xcompile: Add CROSS_COMPILE_* variable

Add CROSS_COMPILE_* variable that can be passed to third-party
component compilations.

BUG=chrome-os-partner:40414
BRANCH=None
TEST=CROSS_COMPILE_* variable is passed in correctly.

Change-Id: I053325524601adf76ea35f7e74811fbc1c31781e
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: c07bd66e25900d064d6c69bddada67112ba5f183
Original-Change-Id: I3e4a5262acd84830205f64ba9a935974dd36ebbd
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/272372
Original-Tested-by: Furquan Shaikh <furquan@chromium.org>
Original-Reviewed-by: Julius Werner <jwerner@chromium.org>
Original-Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Original-Trybot-Ready: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/10306
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>