electronic-lab: add libre FPGA toolchain components
    
    Together, these packages provide a completely free/libre/open FPGA
    toolchain: icestorm and trellis provide architecture databases for
    iCE40 and ECP5 FPGAs, respectively; yosys provides front-end Verilog
    synthesis; finally, nextpnr provides place-and-route and bitstream
    generation capabilities.
    
    Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>