Makefile: workaround make 4.3 issue for 'printvars and 'show-vars'

Event though the bug with make 4.3 has been reported and fixed, there
has not been a release of make with the fix for a long time, see [1].

As the root cause seems the 'filter' command cannot handle large
chunks of data, like .VARIABLES, we can workaround the problem by
using a foreach command over .VARIABLES, then use the filter command.

It might not be logical to program it that way, but at least the
functionality is now usable.

[1] https://savannah.gnu.org/bugs/?59093#comment10



Signed-off-by: Henri Roosen <henri.roosen@ginzinger.com>
[yann.morin.1998@free.fr: add comment to reference the bug]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>