Makefile: update comment about top-level parallel Makefile

After the latest patches top-level parallel Makefile is working but
there is still an issue when a package has an unspecified optional
dependency so change the comment to explain that.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -42,7 +42,20 @@ export HOSTARCH := $(shell uname -m | \
 	    -e s/macppc/powerpc/\
 	    -e s/sh.*/sh/)
 
-# This top-level Makefile can *not* be executed in parallel
+# 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
+# build using the following command:
+#	make BR2_JLEVEL= -j$((`getconf _NPROCESSORS_ONLN`+1))
 .NOTPARALLEL:
 
 # absolute path