build_image: Use BOARD_ROOT to get the sysroot.

Instead of computing the sysroot ourselves from BOARD, use BOARD_ROOT.

BUG=None
TEST=trybots.

Change-Id: I20fa36f202a214fae9020e984890cbe20321706d
Reviewed-on: https://chromium-review.googlesource.com/266032
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
1 file changed