config-parser: export functions to open a config file

The in-tree clients can access the functions via libshared, but they are
currently not available for external clients, such as custom shell helper
applications similar to weston-desktop-shell or
weston-ivi-shell-user-interface.
The functions to read the content of the config are already exported.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>