Add bios support to Archlinux

Add bios support to Archlinux

In order to add support to boot on bios mode for Archlinux, the
following changes were made:

    - kernel packages are now installed as a second step: this allows us
      to patch the initrd config file so to disable the autodetection
      hook
    - patching the initrd config is now part of the distro install
      rather than bootloader install

uefi-only is the default and we can generate uefi-only, bios-only and
hybrid images. In the latter case grub is used for bios and systemd-boot
for UEFI.