soft_i2c.c add option for repeated start in i2c_read()

This patch adds a #define to optionally change the behaviour of
i2c_read() in soft_i2c.c to send an I2C repeated start instead of a
stop-start between sending the device address pointer write and
reading back the data.  The current behaviour is retained as the
default.

While most devices will work either way, I have a smart battery(*)
that requires repeated start, and someone at some point found a
device that required a stop-start.

(*) http://www.inspired-energy.com/Standard_Products/NL2054/NL2054%20Rev1.0%20Data%20Sheet.pdf



Signed-off-by: Andrew Dyer <adyer@righthandtech.com>