Hello!

Long story short: hackrf sink code to convert from GR complex to hackrf 8-bit format fails by generating wrong bits on ARM when I do TX on Raspberry PI 2 .

Evil hides here:
lib/hackrf/hackrf_sink_c.cc
outbuf[i] = inbuf[i]*127;

Since on ARM GCC treats all "char" as unsigned(this is chosen implementation) it generates wrong bits, so my advice is to compile on ARM gr-osmocom with flag -fsigned-char. This is probably most effective way to solve this, it solved my TX problems.