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Changes the default cros_board value in DEPS from "amd64-generic" to the
empty string.

Changes the cros SDK deps to then be additionally gated on cros_board
being non-empty.

This is in preparation for setting target_os=["chromeos"] in gclient
configs for all Chrome OS builders, including desktop Chrome OS,
without causing the latter to pull in unnecessary SDK deps.

Bug: 822034
Change-Id: I04b62c366731e4c18722df45767a2868c10d8b11
Reviewed-on: https://chromium-review.googlesource.com/1036003Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554877}