split -Werror out into a dedicated var
split -Werror out into a dedicated var

This lets us control it explicitly.  In CrOS, we won't turn it off, but
most distros will want to as it's too hard to keep a handle on compilers
and various settings users leverage.

BUG=chromium:466499
TEST=precq still passes
BRANCH=None

Change-Id: I11bf03acefa3fd624dd4c5688dd18e3a3c87647d
Reviewed-on: https://chromium-review.googlesource.com/259521
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

acorn-marvell/vboot_reference,acorn-marvell/vboot_reference,acorn-marvell/vboot_reference,coreboot/vboot,coreboot/vboot,coreboot/vboot,coreboot/vboot,acorn-marvell/vboot_reference,coreboot/vboot