Change the default call sign

[schulz89]

In the code, the default call sign for CW and GMSK packets is from BIRDS-4 TSURU.
Also, the default call sign of destination ground station is the same as Kyutech ground station.
It would be better to change the default call signs to dummy ones to avoid confusion.

[maisunmonowar]

Hey Victor,
O-shashiburi.
I could tackle this over the weekend if you wish.