build_image_util: fix generation of packages.txt

The passing ROOT= as an environment variable to board wrapper scripts
doesn't work, the script unconditionally overrides it. This means so far
our packages.txt files have listed the contents of /build/amd64-usr
instead of the image. Fix this by calling equery directly instead.
Merge pull request #328 from marineam/pkgs

build_image_util: fix generation of packages.txt