hdl: Removing unused signals and fixing NOW.

NOW timestamp was not being considered properly when reading
the metadata FIFO.  This is fixed by throttling the reading of
the metadata FIFO based on the length of the samples.
hdl: Bumped FPGA version to v0.1.2
libbladeRF: Added TX timestamp now/scheduled test

This test exercises switching between the use of the TX NOW flag and
schedule TXs. This was previously an issue (#334) with FPGA <= v0.1.1,
and has been resolved in FPGA v0.1.2.