RAMPS-FD: add SD connector?

[bobc]

Consider adding connector for micro-SD card. SD card can be used for storing config and data files.

uSD can be interfaced directly to Due, but will need level conversion for Mega.

Since an SD card can be easily added with SDRAMPS, it is not essential to have onboard.

Action: defer for now and review later, there may be more important things to add.