allow overloading of recipe origin using env vars

This allows to use env vars to override the download of package for recipes, e.g.

    export P4A_pyjnius_DIR=~/pyjnius

will make python-for-android copy ~/pyjnius instead of dowloading and extracting pyjnius from the url given in pyjnius recipe.

use a bashism, but since the script is declared to use /bin/env bash, shouldn't be a problem.