Because of the (unfortunate) ways that PROPERTIES works currently,
setting defaults in the obvious way means that providing a subset of the
values as properties replaces the defaults entirely.

In this case we want to override the sdk_version without having to
override the toolchain app version.

R=jchinlee@chromium.org, tandrii@chromium.org

Change-Id: I13f4ee19e45a3bb147b37f69d10cb4ed4f734a21
Reviewed-on: https://chromium-review.googlesource.com/c/1438088
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>