archiso/mkarchiso: allow choosing boot modes from profiledef.sh

Boot mode names are:

- bios_syslinux.mbr: SYSLINUX in MBR
- bios.syslinux.eltorito: SYSLINUX (ISOLINUX) via El Torito
- uefi-x64.systemd-boot.esp: systemd-boot on ESP in MBR
- uefi-x64.systemd-boot.eltorito: systemd-boot on ESP via El Torito

It is not yet possible to create an ISO with only El Torito or only MBR boot modes!