While nano specs are certainly handy, it should be documented how to use them,
not turned on by default. People with otherwise valid toolchains, who might
simply not care about nano specs are unable to build the examples with these
options.
This example is a merge of Chuck McManis's example for F4 and onnokort's system clock setting test blink example for demonstration issue, corrected in #224.
The LED in any available clock setting flashes four times per second.
