i2c-explorer: Better console memory mgt.

Console commands provide a scracth area. Its much easier to
manage the scratch area using a structure...

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
i2c-explorer: Add support for I2C3

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>
i2c-explorer: Add i2cwrite command.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>