Added synthesis target

Added synthesis target

The synth target can be used to analyze the core after synthesis
without running P&R. Currently, the only edalize backends that
support synthesis without P&R are vivado and icestorm, and icestorm
needs yosys built with verific support to parse vhdl.

To run synthesis only for a part, run

fusesoc run --target=synth --tool=vivado microwatt --part=<part>

where part is a valid Xilinx part such as xc7a100tcsg324-1