Clean up building code coverage

Now 'COV=1 make' will make coverage automagically.

Coverage stats are reported separately for the firmware subdirectory,
for easier tracking.

BUG=chromium-os:38139
BRANCH=none
TEST='COV=1 make' then browse to build/coverage/index.html

Change-Id: Ie671a82b402beeb17882536f89b9230821fc4d13
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/41824
1 file changed