[autoboot] Avoid excess backspacing after displaying Ctrl-B prompt

Remove the newline from the "Press Ctrl-B..." prompt string, so that
prompt() does not attempt to backspace beyond the start of the line.

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

src/core/main.c

 	if ( BANNER_TIMEOUT <= 0 )
 		return 0;
 
-	return ( prompt ( "\nPress Ctrl-B for the iPXE command line...",
+	/* Prompt user */
+	printf ( "\n" );
+	return ( prompt ( "Press Ctrl-B for the iPXE command line...",
 			  ( BANNER_TIMEOUT * 100 ), CTRL_B ) == 0 );
 }