Added legacy pin coverager for nano boards

[mjs513]

While playing with SDfat kept getting an error about ss pin not defined. Then @KurtE reminded that on the teensy they are defined in the core. Then remembered say that for the giga. When I looked for the nano boards (variant.h) they were not covered. This PR adds:

// TODO: correctly handle these legacy defines
#define MOSI    0
#define MISO    0
#define SCK     0
#define SS      0
#define SDA     0
#define SCL     0

to the nano variant.h files (ble, sense and iot).

recompiled core and retested and the sdfat compile error disappeared.

[mjs513]

after testing sdfat had to actually specify pins for SPI which we didn't have to do for GIGA spi.

@facchinm - @KurtE: Not sure if we should specify pins for SDA/SCL as well since NANO is different that giga it seems