...
DEFAULT arch
PROMPT 0
SERIAL 0 38400
TIMEOUT 50
...
LABEL arch
    MENU LABEL Arch Linux
    LINUX ../vmlinuz-linux
    APPEND root=/dev/disk/by-label/root rw console=ttyS0,38400n8
    INITRD ../initramfs-linux.img

LABEL archfallback
    MENU LABEL Arch Linux Fallback
    LINUX ../vmlinuz-linux
    APPEND root=/dev/sda3 rw console=ttyS0,38400n8
    INITRD ../initramfs-linux-fallback.img
...