unit tests: support code coverage instrumentation and reports

Currently, make check runs the unit tests and reports pass/fail,
but we have no way to guage how much of the code is covered by
these tests.  gcov provides those statistics on a per source
file basis, but requires special CFLAGS and LDFLAGS.  Here, we
add the --enable-gcov configure option to setup those options
correctly.  We also add a make target called check-coverage,
which runs the unit tests, runs gcov and uploads the data to
the codecov.io cloud service for display.

Finally, we include a Dockerfile-coverage which creates a
container image in alpine linux to run the tests.  To create
the image:

$ docker build \
	--build-arg commit=`git rev-parse HEAD` \
	--build-arg token=<upload token from codecov.io> \
	-t frr-gcov:latest \
	-f docker/alpine/Dockerfile-coverage .

and to create and upload the report:

$ docker run -it --rm frr-gcov:latest

Testing done:

Created and uploaded a report from my fork using alpine linux 3.7.
Non-coverage alpine 3.7 build still works.

Issue: https://github.com/FRRouting/frr/issues/2442
Signed-off-by: Arthur Jones <arthur.jones@riverbed.com>

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