cdrom.eclass: Make CD_ROOT less of a special case, fixes #195868

cdrom.eclass: Make CD_ROOT less of a special case, fixes #195868

CD_ROOT was intended to be a power user feature so the eclass didn't
try very hard to check the validity of the given location. This
difference in behaviour ultimately made the eclass larger and more
confusing.

It now uses the same matching loop as the regular case, making it
simpler and more consistent. The only differences are that it doesn't
show information or prompts about inserting discs and it dies
immediately if a match cannot be found.