[arch-releng] giving archiso block devices more clear names

Hi, to solve https://bugs.archlinux.org/task/25425 in a robust and clear way, I suggest to - in archiso - prefix all blockdevice filenames with "archiso_". (also, I'm more a fan of underscores then hyphens, in this case it might not matter, but they are usually a bit more robust (like LVM filenames and for variables in bash) so I stick to them for consistency) I.e. /dev/mapper/archiso_lib_modules /dev/mapper/archiso_root_image /dev/mapper/archiso_usr_share Dieter