Accept "source" as an alternative to "recipe"

When building from binaries, you can now use pkg = "source" to specify that a single package should be built from the recipe. The feature was already supported, but you would say pkg = "recipe". Both "source" and "recipe" are now supported.

So, you can say in your config file pkg = "binary" to download the package from the web, and pkg = "source" or pkg = "recipe" to always build the recipe. These values override the value of REPO_BINARY, which determines what to do if the config file specifies pkg = {}.