The first iterations of wic very shortsightedly catered to two
specific use-cases and added special-purpose params for those cases so
that they could be directly given their corresponding boot artifacts.
(hdddir and staging_data_dir).

As more use-cases are added, it becomes rather obvious that such a
scheme doens't scale, and additionally causes confusion for plugin
writers.

This removes those special cases and states explicitly in the help
text that plugins are responsible for locating their own boot
artifacts.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

As well as any other stray instances of mic in the codebase that can
be removed.

We don't really need to carry around legacy naming, and the history is
in git.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>