Makefile: work around a bug in newly released make 4.3

Several users of rolling-release distributions have been reporting on
IRC that Buildroot is broken now that they have switched to the newly
released make 4.3.

It turns out that the constructs we use to generated and include the
internal br2-external related fragments is no longer working with
make-4.3.

Indeed, an upstream bug report [0] seems to imply that it so far was
working by chance. There has been no further feedback, whether this is
really considered a fix for a previous ill-defined behaviour, or an
actual regression...

In the meantime, we add a workaround, suggested in that same bug report,
that fixes the issue for make 4.3, and that should not break on older
make versions either (verified on all relevant versions: from 3.81,
3.82, 4.0, 4.1, and 4.2).

[0] https://savannah.gnu.org/bugs/?57676



Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Mircea Gliga <mgliga@bitdefender.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>