uhd tool to send gmsk/8psk bursts while synchronously toggling the first gpio pin of a b2xx.

sends one burst per second.

example usage:
```shell
cd burst-sender
mkdir build
cd build
cmake ..
make -j
cd ..
build/burstelchen -burstpath ./gmsk_tsc0/ -extref -txgain 50
```

requires uhd to build.