This allows "derived" libraries (like the one in Arduino101 core) to use both the onboard or an external flash, while avoiding forks

Still not sure about SPIClass, is it universal? I coundn't find a core declaring it differently but I'm quite sure it exists. "typeof" SPI would be better.

it checks for all bytes=0 or 0xff (the pin might be hold low or high by other connections)

Fixes this problem:
https://forum.pjrc.com/threads/33594-SerialFlash-filesystem-question?p=101511&viewfull=1#post101511

(inportant, i think...)

https://forum.pjrc.com/threads/33328-Prop-Shield-Beta-Test?p=98553&viewfull=1#post98553

The Spansion cards can have 64Kb or 256Kb depending on the model's
number. The device id[4] must be checked to determine the sector
architecture.

Shift was wrong direction