cmake: Skip libc & libusb version checks when cross-compiling

These checks break cross-compilation, as they require code to be
run on the host machine.
Initial ATSC TX capability with a random data generator in the top level.
Cleanup bad comments, remove unused variables/signals.  Add in raw binary type for the cli.
RAW file type will no longer cause an assert.  Small atsc_tx cleanup.
Fix a whitespace error in the build script.
Fix a bug when shifting bits out of the stripper.  Update the scaling of the pilot in the atsc_tx block.  TB cleanup.
Change symbol width sent over USB from 3 bits to 4, this will help maintain alignment.  Fix an off by one error in the phase incrementing when centering.