core: add local files to fetched core
core: add local files to fetched core

Core can now be a mixture of remote files _and_ local files.
If a file exist in the remote location it can be overwritten by
the local version.

Signed-off-by: Franck Jullien <7ddcd52d897de62c0010070b24d1f61cf69e2d93@gmail.com>

olofk/fusesoc,lowRISC/fusesoc,olofk/fusesoc,lowRISC/fusesoc