Fix archiso bootmodes and add syslinux to package list

mkarchiso validation: bios.syslinux.mbr/eltorito and uefi-x64.* bootmodes
are deprecated -> use bios.syslinux + uefi.systemd-boot. syslinux must be
in the package list for the BIOS bootmode; add memtest86+/edk2-shell too.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/profiledef.sh

@@ -8,12 +8,7 @@ iso_application="Bread Operating System"
 iso_version="$(date +%Y.%m.%d)"
 install_dir="arch"
 buildmodes=('iso')
-bootmodes=(
-    'bios.syslinux.mbr'
-    'bios.syslinux.eltorito'
-    'uefi-x64.systemd-boot.esp'
-    'uefi-x64.systemd-boot.eltorito'
-)
+bootmodes=('bios.syslinux' 'uefi.systemd-boot')
 arch="x86_64"
 pacman_conf="pacman.conf"
 airootfs_image_type="squashfs"