check conditions of  make  for equality of variables with an empty value
instead of check definition.
A defined but empty variable under all these conditions results errors.

Duplicating the architecture tag handling logic was afwul
indeed; one-liner (well, two lines for readability) is not
that bad at least, factoring it out wouldn't gain a lot.

Fixes: 75735473

Allow placing profiles in directory like groups and lists.

See commit 2d257a42
for details, this just brings those to pkg.in/profiles/
(along with the generic archdep support, of course).

...to reduce the divergence with the rest of related variables;
these installer profiles affect the default checkbox states.

This is to support Centaurus-like images featuring
more than a single installation profile (e.g.,
"workstation/server/minimal").

Quite a few more modifications are needed to do it
right though as m-p has fallen prey to exactly the
"single root composition" assumption with its current
THE_* variables semantics and processing.