Make SYSROOT a portage variable

SYSROOT should be a special variable and not get included in the binary
package's environment.

BUG=chromium:404018
TEST=inspect environment.bz2 from a binary package. SYSROOT is not defined
there.
TEST=trybot run on daisy, duck, link, gizmo, mispel32, chromiumos-sdk.

Change-Id: I2b93bd37dda2263e7ccb2ccf9e7f9963a4fa5bc7
Reviewed-on: https://chromium-review.googlesource.com/221343
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
2 files changed