baseline: Add buildmodes to profiledef.sh

configs/baseline/profiledef.sh:
Add `buildmodes` array with default entry for the 'iso' buildmode.

configs/baseline/profiledef.sh

@@ -7,6 +7,7 @@ iso_publisher="Arch Linux <https://archlinux.org>"
 iso_application="Arch Linux baseline"
 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')
 arch="x86_64"
 pacman_conf="pacman.conf"