Validate profile by checking for required files and packages

• Tidy up various cruft.

• Validate profiledef.sh (fixes #76 (closed)):

  • check if the files packages.${arch} and pacman_conf exist,
  • check if there is more than one package listed,
  • check if the specified bootmodes are supported and if their requirements are met,
  • check if the specified airootfs_image_type is supported and its requirements are met.

• Don't hardcode boot mode names in _build_iso.