synth_sf2: add -discard-ffinit option to discard ff initial value
synth_sf2: add -discard-ffinit option to discard ff initial value

sf2 ff have no initial values, but some IP cores use initial values.
In order to use those cores on sf2, it is required to discard the
initial value (to be carefully used).

YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys,YosysHQ/yosys