The original _getkey does the following:

1. wait until UART receiver (hardware) signals "character ready" (by setting RI)
   
2. reads that character from the receiver
   
3. clears the flag
   
4. returns the character

You have an interrupt where you scan the keyboard. So, treat this interrupt as the "hardware", let it set a flag "character ready" and store the received (and decoded) character in a global variable.

Then, rewrite getkey, to do absolutely same, but with YOUR flag instead of RI, and with reading the character of said global variable.

Note, that you will need to decode some of the keys into CR (ENTER, \n, 0x0D, 13, whichever you like).

Now, try this, while having still the original serial output for printf (I assume you already have some serial connection, terminal emulator on PC etc>).

Later, you can replace the putch so that the output will go to LCD rather than the UART.

JW