The new function factory_reset_make_files() populates the given filesystem
with the default factory files. It is defined with weak linkage so it can
be overridden by a board.
This commit also brings some minor user-facing changes:
- boot.py is now no longer created unconditionally if it doesn't exist, it
is now only created when the filesystem is formatted and the other files
are populated (so, before, if the user deleted boot.py it would be
recreated at next boot; now it won't be).
- pybcdc.inf and README.txt are only created if the board has USB, because
they only really make sense if the filesystem is exposed via USB.
