Automatically boot the installation environment from syslinux

Timeout is 3 seconds to match systemd-boot's configuration.

configs/baseline/syslinux/syslinux.cfg

@@ -2,6 +2,9 @@ UI boot/syslinux/menu.c32
 MENU TITLE Arch Linux
 MENU CLEAR
 
+DEFAULT arch
+TIMEOUT 30
+
 LABEL arch
 MENU LABEL Arch Linux
 LINUX boot/%ARCH%/vmlinuz

configs/releng/syslinux/archiso_sys.cfg

 INCLUDE boot/syslinux/archiso_head.cfg
 
+DEFAULT arch64
+TIMEOUT 30
+
 LABEL arch64
 TEXT HELP
 Boot the Arch Linux install medium on BIOS.