src: implement config file lookup and initial parsing

This commit prepares the use of device-specific configuration files. 
These files should be named after the device-tree `compatible` string 
with the `toml` extension.

`eg25-manager` will search for config files in 
`<prefix>/etc/eg25-manager` first (or `/etc/eg25-manager` if prefix is 
`/usr`), then in `<prefix>/share/eg25-manager`.

src/manager.h

@@ -11,6 +11,8 @@
 #include <gudev/gudev.h>
 #include <libmm-glib.h>
 
+#include "toml.h"
+
 enum EG25State {
     EG25_STATE_INIT = 0,
     EG25_STATE_POWERED, // Power-on sequence has been executed, but the modem isn't on yet