Nginx support

## Notes
This adds nginx support to rootbuilder. I've added a `package/` dir, which additional packages can be added to in future. Should buildroot add official nginx support in the future, the changes to the `Dockerfile` and `nginx/`'s presence in the `package/` directory should be removed.

## Usage
Nginx can be found in the `Target Packages` menu under `Networking applications`. It either requires `glibc` (rather than `uClibc`) or `large file support` under `Toolchain`. If you're having trouble accessing the full nginx submenu (What should be `--->` appears as `---`), try pressing `SPACE` instead of `ENTER` to activate the nginx and the submenu.

## Testing
The best way to test this pull is by building this repository with the tag `progrium/rootbuilder` locally, and running `make config` from the `rootfs/` directory of @progrium 's `busybox` repository. Ensure that you have specified the valid target architecture and that you followed the usage notes above, and run `make` from `busybox`s top-level directory. Rootbuilder should build `rootfs.tar` successfully, and `busybox` should be built successfully also. 

You may need to manually create the `/var/lib/nginx/body` directory before `nginx` works from inside a container, but that's outside the scope of this pull.