M350 changes steps/mm of axes/extruders assigned to CAN-connected boards

[chrishamm]

M350 cannot be used to set the microstepping of locally-connected external drivers as on the 6XD, yet this is permitted for remotely-connected drivers like the 1XD. Like for locally-connected external drivers it should not be permitted to set anything but the x16 (pseudo-)microstepping.

An exception may be the Mini 5+ 2XD because RepRapFirmware does not determine if a Mini 5+ 2+ or a Mini 5+ 2XD is plugged in, hence it's difficult to apply that limitation. In that case the documentation needs to be updated.

[T3P3]

Provide a warning that the M350 does set microstepping on external drivers. Also notify user in the warning that the steps/mm has been updated.

Needs to be consistent with 6XD and 2XD

M350 should return N/A for all external drivers.

Also M906, M913, M915, M917 etc should provide a warning when the current or other non applicable setting is set. (and provide N/A when queried if all the drivers for the axis are external)

This is an incompatible change because we plan to update the microstepping when M350 is sent to handle a mixture of internal and external drivers, so deferred to 3.7

[T3P3]

Possibly related to #784 which would allow an enumeration phase on start-up.