Make the generation of IPBus address decoders respect addr_table dir substructure

[jhegeman]

Now, when generating the IPBus address decoder for a file addr_table/XXX/a.xml, the generated decoder VHDL file will be firmware/hdl/XXX/ipbus_decoder_a.vhd. (The previous behaviour would ignore the 'XXX' substructure of the addr_table directory.)

[alessandrothea]

LGTM