[shell_banner] Avoid printing Ctrl-B prompt if BANNER_TIMEOUT <= 0

Signed-off-by: Marty Connor <mdc@etherboot.org>

src/hci/shell_banner.c

 	int wait_count;
 	int key;
 
+	if ( BANNER_TIMEOUT <=  0 ) {
+		return enter_shell;
+	}
+
 	printf ( "\nPress Ctrl-B for the gPXE command line..." );
 
 	/* Wait for key */