Some of our customers encountered clock stretching issue when using the I2C. Actually the first should been in mind is the clock stretching is usually done by the slave, not the master.
In the case for the Kinetis as master to connect with I2C device, the clock stretching should been done by the slave device. In this case the slave device should hold the clock signal low until it has data available. There isn’t anything that needs to be done to enable clock stretching on the master side.
So how the code do with the clock stretching? The slave should toggling read_start high first and reading the data register to actually start the transfer. Be remember that the read from the data register is what actually triggers the transfer. Customer always missed this point.