rfnoc: constants.hpp no longer configured at build time
rfnoc: Introduced separate rx and tx channels
rfnoc: Map subdev spec to channel definition
x300: make_transport now distinguishes chan types again
x300/rfnoc: Clean up some crufty code
rfnoc: Introduced device3/ subdir + device3_impl class
e300: rfnoc: Initial commit for E300 rfnoc support.

Signed-off-by: Moritz Fischer <moritz@ettus.com>
e300: Refactored DMA channel mapping.

Note: This assumes a crossbar layout starting
      with {AXI, R0, R1}

Signed-off-by: Moritz Fischer <moritz@ettus.com>