[rpm] What is the minimum I have to do to create an RPM file?

For quick RPM building, check out Togo:

https://github.com/genereese/togo-rpm

The project has a Quick-Start guide and I was able to create a basic RPM in less than 3 minutes.

Example using the data provided in the original question:

1) Create the project directory using the script:

$ togo project create foobar; cd foobar

2) Make your desired directory structure under ./root and copy your files into it:

$ mkdir -p root/etc; cp /path/to/foobar.conf root/etc/
$ mkdir -p root/usr/bin; cp /path/to/foobar root/usr/bin/

3) Exclude system-owned directories from your RPM's ownership:

$ togo file exclude root/etc root/usr/bin

4) (OPTIONAL) Modify the generated spec to change your package description/dependencies/version/whatever, etc.:

$ vi spec/header

5) Build the RPM:

$ togo build package

-and your RPM is spit out into the ./rpms directory.