Refine gdb.in, no more operations about C-style string.
modified:   gdb.in

1. Retrieve the length of variable and utilize it to send char to UART.
2. Utilize GDB command "dump" to dump the content of variable to file.

modified:   main.c

In order to retrieve the length of variable in GDB session, I have to
declare the C-style string with char array, but char pointer; otherwise
sizeof with a pointer will return 4.