define platform.soc_ext_path when --external argument is used (to use it to include verilog files in the external target)
tools: replace mkmscimg with mkmscimg.py (mkmscimg.c was platform dependent)
Refactor CRC tools
lasmicon/lasmibus: simplify dat_ack by splitting it in dat_w_ack and dat_r_ack.
dat_w must be presented when dat_w_ack is set.
dat_r is valid when dat_r_ack is set.

In the old version, write_latency and read_latency on data was managed
directly in the ports which was not easy to do.
make.py: add missing --csr_csv argument