Clear the screen after syslinux exits

Reduce the time where boot loader artifacts linger on screen after the boot loader has done its job.

configs/baseline/syslinux/syslinux.cfg

 UI boot/syslinux/menu.c32
 MENU TITLE Arch Linux
+MENU CLEAR
 
 LABEL arch
 MENU LABEL Arch Linux

configs/releng/syslinux/archiso_head.cfg

@@ -23,3 +23,5 @@ MENU COLOR timeout_msg  37;40   #80ffffff #00000000 std
 MENU COLOR timeout      1;37;40 #c0ffffff #00000000 std
 MENU COLOR msg07        37;40   #90ffffff #a0000000 std
 MENU COLOR tabmsg       31;40   #30ffffff #00000000 std
+
+MENU CLEAR