Standardize boot menu entries

archiso's boot loader menu entries differ between boot loaders (and boot mode):

• SYSLINUX: Boot Arch Linux (x86_64)
• PXELINUX: Boot Arch Linux (x86_64) (NBD/NFS/HTTP).
• systemd-boot on FAT: Arch Linux archiso x86_64 UEFI USB
• systemd-boot on ISO 9660: Arch Linux archiso x86_64 UEFI CD

All of them should use some common standard.