Allow passing defines to symbiflow_synth

[carlosedp]

Running as:

synth.f4pga.sh -v myfile.v -x constr.xdc -t Toplevel -d xc35 -p 384 -e DEFINE_1_=1 DEFINE_2_=2

Generates: yosys -p ; tcl /Users/cdepaula/repos/f4pga/f4pga/wrappers/tcl/xc7.f4pga.tcl -l Toplevel_synth.log -DDEFINE_1_=1 -DDEFINE_2_=2 myfile.v