core: Return a file descriptor from open_config()

core: Return a file descriptor from open_config()

Wrap open_config() the same way that open() wraps open_file(). The
only user of open_config() requires access to a file descriptor so it
makes sense to return a file descriptor.

The file handle implementation is a historic piece of code and this
patch tries to hide it as they will likely be removed at a future
point in time. Furthermore, the file handle code is very core-specific
and should not be exposed to any callers of open_config().

Signed-off-by: Matt Fleming <matt.fleming@linux.intel.com>