IO_MANAGER: early dynamic driver registration

Adds the possibility to register drivers before the IO Manager has
completed the initialization. Sometimes the BSP may want to register
a driver dynamically early in the boot process.

cpukit/sapi/src/io.c

@@ -21,10 +21,14 @@
 
 #include <rtems/io.h>
 
+int _IO_Manager_drivers_inititalized = 0;
+
 void _IO_Initialize_all_drivers( void )
 {
    rtems_device_major_number major;
 
+   _IO_Manager_drivers_inititalized = 1;
+
    for ( major=0 ; major < _IO_Number_of_drivers ; major ++ )
      (void) rtems_io_initialize( major, 0, NULL );
 }

cpukit/sapi/src/ioregisterdriver.c

@@ -25,6 +25,8 @@
 #include <rtems/rtems/intr.h>
 #include <rtems/score/threaddispatch.h>
 
+extern int _IO_Manager_drivers_inititalized;
+
 static inline bool rtems_io_is_empty_table(
   const rtems_driver_address_table *table
 )
@@ -108,5 +110,15 @@ rtems_status_code rtems_io_register_driver(
 
   _Thread_Enable_dispatch();
 
-  return rtems_io_initialize( major, 0, NULL );
+  if ( _IO_Manager_drivers_inititalized ) {
+    /* Other drivers have already been initialized, we initialize
+     * the driver directly.
+     */
+    return rtems_io_initialize( major, 0, NULL );
+  } else {
+    /* The driver will be initialized together with all other drivers
+     * in a later stage by _IO_Initialize_all_drivers().
+     */
+    return RTEMS_SUCCESSFUL;
+  }
 }