Blinken Button SPI
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Adds SPI support to the Blinken Button by Interactive Matter[1].
This allows the 8x8 LED display to be controlled by another MCU.
The Blinken Button receives frames from an SPI master, which are shown
on the display. A frame consists of 8 bytes, each of which represents
rows 0 to 7.
[1] http://interactive-matter.org/how-to/blinken-button-for-beginners/