core/out-of-tree: fix Makefile wrapper

Commit 971faf82 (Makefile: fix out-of-tree builds with multiple targets
with 'all') renamed the default target to '_all' to avoid name-clashing.

In doing so, I forgot to also fix the instance in the .PHONY rule.

Fix that now.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>