Functions UI build settings

[ehmicky]

The following line:

https://github.com/netlify/netlify-plugin-nextjs/blob/7b9a2430f46da57a38c5c85d611b2fc7df6bd777/index.js#L59

Is checking the build.functions configuration property in netlify.toml. However, constants.FUNCTIONS_SRC should be used instead, since it takes more cases into account, such as when users are creating functions directory during build.

[ehmicky]

Done at #28.