delay_bram mod for pfb_fir

pfb_fir blocks are not created by script and the default delay_bram
has changed to being empty, this caused unconnected nets. delay_bram
changed back to being populated and pfb library updated (again) to
not have unconnected nets by default.
DSP48 implementation option for bus_addsub

Added to allow logic saving in FFT
Added dsp48 adder logic to butterfly_direct script

Can now choose to implement twiddle logic using DSP48s instead
of fabric