[init] Remove concept of "shutdown exit flags"

Remove the concept of shutdown exit flags, and replace it with a
counter used to keep track of exposed interfaces that require devices
to remain active.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/device.c

@@ -35,6 +35,9 @@ FILE_LICENCE ( GPL2_OR_LATER );
 /** Registered root devices */
 static LIST_HEAD ( devices );
 
+/** Device removal inhibition counter */
+int device_keep_count = 0;
+
 /**
  * Probe a root device
  *
@@ -87,11 +90,11 @@ static void probe_devices ( void ) {
  * Remove all devices
  *
  */
-static void remove_devices ( int flags ) {
+static void remove_devices ( int booting __unused ) {
 	struct root_device *rootdev;
 	struct root_device *tmp;
 
-	if ( flags & SHUTDOWN_KEEP_DEVICES ) {
+	if ( device_keep_count != 0 ) {
 		DBG ( "Refusing to remove devices on shutdown\n" );
 		return;
 	}

src/core/main.c

@@ -85,7 +85,7 @@ __asmcall int main ( void ) {
 			shell();
 	}
 
-	shutdown ( SHUTDOWN_EXIT | shutdown_exit_flags );
+	shutdown_exit();
 
 	return 0;
 }