grlib: Add ambapp_plb()

Replace the global variable ambapp_plb with a function to allow an automatic on
demand initialization.

bsps/include/grlib/ambapp.h

@@ -188,6 +188,11 @@ struct ambapp_context {
   void *(*alloc)(size_t);
 };
 
+/**
+ * @brief Gets the fully scanned AMBA Plug & Play Processor Local Bus (PLB).
+ */
+extern struct ambapp_bus *ambapp_plb(void);
+
 /* Scan a AMBA Plug & Play bus and create all device structures describing the 
  * the devices. The devices will form a tree, where every node describes one
  * interface. The resulting tree is placed in the location pointed to by root.

bsps/include/grlib/grlib_impl.h

@@ -157,8 +157,6 @@ RTEMS_INLINE_ROUTINE unsigned int grlib_read_uncached32(unsigned int address)
 
 #endif
 
-extern struct ambapp_bus ambapp_plb;
-
 #ifdef __cplusplus
 }
 #endif