Merged r7611:7614 from jcorgan/gpio into trunk. Adds custom
FPGA build for streaming digital I/O to/from GPIO pins through LSB of I and Q
datastreams, with example programs of use.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@7618 221aa14e-8319-0410-a670-987f0aec2ac5