Why Freescale doesn't have an implementation of an interrupt driven UART?
I would use the Async UART bean of Processor Expert. I highly reccomend processor expert as you can develop things orders of magnitude faster than looking through the chips header files for various flags and masks.
There are "on recieved" interrupts that you can enable that will direct execution to these interrupt routines when a char is accepted in the RX buffer. Then all you have to do is check until some signal is sent by the PC that you can use as an end of transmission character, or wait until you receieve a set quantity of bytes, or just wait a set amount of time. Then send the data needed back to the PC. If you need a relatively simple protocol to help organize your data take a look at Modbus RTU. It's very easy to implement, and will save you time if you have to reorganize data/send more data in the future.
Regards,
Cory