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

[Dieter Plaetinck]

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