This fixes the following compile error on Mac:
src/drivers/gps/gps.cpp:562:23: fatal error: use of undeclared identifier 'B921600'
case 921600: speed = B921600; break;
^
The _MAV_PAYLOAD macro casts the value to a char*. Some compilers treat
char as signed, so we were getting negative system and component ids
if the id was >= 128. This caused later comparisons to fail which
resulted in messages not being forwarded.
This fixes running the Ubuntu setup script in a Python virtual
environment. This was failing because pip doesn't allow the --user
option in virtual environments.
David Lechner