use ${} with make vars instead of $()

BUG=None
TEST=make still builds
BRANCH=none

Change-Id: Ie7292e4cc88338dc6544fd859028ce55557578b4
Reviewed-on: https://chromium-review.googlesource.com/241043
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@google.com>
1 file changed