Add a make target to check for clean git status
Add a make target to check for clean git status

The `valid-git-status` target will fail the build if the git status
is not clean, i.e. if there are any modified or untracked files.

Change-Id: I158049ee7c730ce2d1ec6acd946c908bc556dcae

dpursehouse/pygerrit2,dpursehouse/pygerrit,sonyxperiadev/pygerrit,markon/pygerrit2,gferon/pygerrit2,morucci/pygerrit,benjiii/pygerrit