adding spi retail and spi dev support

[al3x10m]

not an issue..but i wonder if you guys could add support for b9s spi boot support...for those crazy blokes that replaced the spi flash on their wifi board.
if not...well..for completeness' sake :D
(tested b9s flashed to spi and it boots)

[SciresM]

I [whatever the opposite of actively maintain is] this project is, these days.

Make a pull request, and be sure to test it before doing so?