seminaries/qtrvsim/uart-calc-add: task to receive two decimal numbers from UART and send sum in decimal to the UART.

Implement receive, add and print functions and place C language source
file on path

  work/uart-calc-add/uart-calc-add.c

To run the code in qtrvsim_cli, the serial port to file redirection
can be used, see the Makefile.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>