New 'available' function in device API, to determine whether required ROMs are available. Currently used when building soundcard menu, to remove AWE32 when required ROM dump is not present.

src/device.h

@@ -3,6 +3,7 @@ typedef struct device_t
         char name[50];
         void *(*init)();
         void (*close)(void *p);
+        int  (*available)();
         void (*speed_changed)(void *p);
         void (*force_redraw)(void *p);
         int  (*add_status_info)(char *s, int max_len, void *p);