Implement SPI flash timeout

[charlespax]

On Electronics version 0.9 the SPIFlash library hangs on flash.initialize() because it calls SPI.transfer(0), which always returns 255 with IC not present on this hardware version. This is a problem with implementing a SPI flash test in the T400 testing firmware (https://github.com/PaxInstruments/t400-firmware/issues/125).

There are a few options for working around this.

• We can implement a hardware solution involving pullup or pulldown resistors. We would have to review the SPI devices and make sure a solution is compatible with all of them. See the discussion an http://www.microchip.com/forums/m148739.aspx
• We can modify the Arduino SPI library to have a timeout. We would modify SPI.transfer(0). This would require us to distribute yet another library. Maybe we can get a patch upstream, but that sounds very slow.
• We can implement a version of flash.initialize() with a timeout in the SPIFlash library. It might look like flash.initialize(TIMEOUT). If we implement this nicely, we can probably get it pulled upstream.
• We may be able to include something in the t400-firmware that redefines the initialize() function or one of the functions between that and the one that hangs, SPI.transfer(0). This would be the best option as it does not require anything outside of our direct implementation.

[charlespax]

This is a problem I ran into while making test code. Would this be an issue with the shipping firmware?

[cibomahto]

Moving to icebox, reconsider it if you're making them in 10k quantity.