defining $EBVAR env vars should be deprecated/dropped

[boegel]

the motivation for having these around is pretty broken:

from easybuild/tools/toolchain/toolchain.py:

442             # also set unique named variables that can be used in Makefiles
443             # - so you can have 'CFLAGS = $(EBVARCFLAGS)'
444             # -- 'CLFLAGS = $(CFLAGS)' gives  '*** Recursive variable `CFLAGS'
445             # references itself (eventually).  Stop' error
446             setvar("EBVAR%s" % key, val)

[wpoely86]

The := assignment expanded immediately so when you do something like:

A =
CFLAGS = -Wall ${A}

where A gets filled in later in a makefile, you cannot switch to :=. To avoid recursive issues, you need a EBVAR variable.

[boegel]

@wpoely86 Do you mind issuing a PR to clarify the comment why $EBVAR variables are also being defined, and to add a test to check whether they're being set (so we don't break this accidentally)?

Worth mentioning: $EBVARCFLAGS & co are required for the patch for FSL added in easybuilders/easybuild-easyconfigs#4614