the directory argument for generic_kv_ro should be passable at configure time

[yomimono]

e.g. mirage configure -t unix --dir="my_rad_dir", suggested by @avsm.

[hannesm]

I'm interested in this as well. at the moment, for a generic_kv_ro we pass the directory argument hardcoded in config.ml, which is certainly not the right thing to do. instead, it should be an (configuration-time) argument where the default value can be hardcoded in config.ml. I tried to modify the crunch device in such a way, but got stuck (please see https://github.com/hannesm/functoria-playground/tree/master/kv-ro-configure how far I got and where I'm stuck). maybe @Drup has an idea how to move forward.