Makefile: allow top-level parallel build with BR2_PER_PACKAGE_DIRECTORIES=y

With per-package folder support, top-level parallel build becomes
safe, so we can enclose the .NOTPARALLEL statement in a
!BR2_PER_PACKAGE_DIRECTORIES condition.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -227,21 +227,12 @@ ifeq ($(filter $(noconfig_targets),$(MAKECMDGOALS)),)
 -include $(BR2_CONFIG)
 endif
 
-# Parallel execution of this Makefile is disabled because it changes
-# the packages building order, that can be a problem for two reasons:
-# - If a package has an unspecified optional dependency and that
-#   dependency is present when the package is built, it is used,
-#   otherwise it isn't (but compilation happily proceeds) so the end
-#   result will differ if the order is swapped due to parallel
-#   building.
-# - Also changing the building order can be a problem if two packages
-#   manipulate the same file in the target directory.
-#
-# Taking into account the above considerations, if you still want to execute
-# this top-level Makefile in parallel comment the ".NOTPARALLEL" line and
-# use the -j<jobs> option when building, e.g:
-#      make -j$((`getconf _NPROCESSORS_ONLN`+1))
+ifeq ($(BR2_PER_PACKAGE_DIRECTORIES),)
+# Disable top-level parallel build if per-package directories is not
+# used. Indeed, per-package directories is necessary to guarantee
+# determinism and reproducibility with top-level parallel build.
 .NOTPARALLEL:
+endif
 
 # timezone and locale may affect build output
 ifeq ($(BR2_REPRODUCIBLE),y)