fix dev server build of certain packages
fix dev server build of certain packages

cd to package directory before invoking make_pkg.sh--a la
build_platform_packages; this fixes building packages that assume
make_pkg.sh is run in the top-level dir

Review URL: http://codereview.chromium.org/516012

coreos/dev-util,marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util,coreos/dev-util,marineam/coreos-dev-util,mgireesh05/dev-util