Set bootloader timeouts to 15s

configs/releng/efiboot/loader/loader.conf,
configs/releng/syslinux/archiso_sys.cfg:

Set the bootloader timeout to 15s, as they have been set to an overly generous 30s in !79.

Fixes #80

configs/releng/efiboot/loader/loader.conf

 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-timeout 30
+timeout 15
 default archiso-x86_64-linux.conf

configs/releng/syslinux/archiso_sys.cfg

@@ -4,7 +4,7 @@
 INCLUDE boot/syslinux/archiso_head.cfg
 
 DEFAULT arch64
-TIMEOUT 300
+TIMEOUT 15
 
 INCLUDE boot/syslinux/archiso_sys-linux.cfg