core: split variables definition related to in/out-of-tree build from O itself

This change uncorrolates the CONFIG_DIR and NEED_WRAPPER definition from
the presence of the O variable in the command line.

Now, the condition used to set these variables is the value of O itself.

This change is a preparatory work since the O definition will need to
be moved around when we will make Buildroot run with absolute canonical
paths for both its root directory and the output location.
This will be addressed in a follow-up patch.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -109,10 +109,11 @@ endif
 # Include some helper macros and variables
 include support/misc/utils.mk
 
+# Set O variable if not already done on the command line;
+# or avoid confusing packages that can use the O=<dir> syntax for out-of-tree
+# build by preventing it from being forwarded to sub-make calls.
 ifneq ("$(origin O)", "command line")
 O := output
-CONFIG_DIR := $(TOPDIR)
-NEED_WRAPPER =
 else
 # other packages might also support Linux-style out of tree builds
 # with the O=<dir> syntax (E.G. BusyBox does). As make automatically
@@ -125,9 +126,16 @@ MAKEOVERRIDES =
 # Unfortunately some packages don't look at origin (E.G. uClibc 0.9.31+)
 # To really make O go away, we have to override it.
 override O := $(O)
-CONFIG_DIR := $(O)
 # we need to pass O= everywhere we call back into the toplevel makefile
 EXTRAMAKEARGS = O=$(O)
+endif
+
+# Set variables related to in-tree or out-of-tree build.
+ifeq ($(O),output)
+CONFIG_DIR := $(TOPDIR)
+NEED_WRAPPER =
+else
+CONFIG_DIR := $(O)
 NEED_WRAPPER = y
 endif