core/out-of-tree: fix Makefile wrapper

Commit 971faf8 (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>

support/scripts/mkmakefile

@@ -30,7 +30,7 @@ MAKEARGS += O=\$(if \$(patsubst /%,,\$(makedir)),\$(CURDIR)/)\$(patsubst %/,%,\$
 
 MAKEFLAGS += --no-print-directory
 
-.PHONY: all \$(MAKECMDGOALS)
+.PHONY: _all \$(MAKECMDGOALS)
 
 all	:= \$(filter-out Makefile,\$(MAKECMDGOALS))