Migrate to and use get_layout_partition_number

The disk_layout_util.sh function get_partition_number collides with a
function of the same name in chromeos-common.sh. Update the name to
avoid the collision, and use the function as needed.

BUG=b:35045101
TEST=`for B in x86-generic amd64-generic;do ./setup_board --board=$B; \
./build_packages --board=$B; ./build_image --board=$B; done` succeeds.

Change-Id: I72a167634471355b03910d04581924da14e64d26
Reviewed-on: https://chromium-review.googlesource.com/450832
Commit-Ready: Ian Coolidge <icoolidge@google.com>
Tested-by: Joshua Emele <jemele@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
5 files changed