package/Makefile.in: expose CONFIG_DIR to post-build/post-image scripts

Sometimes, post-build or post-image scripts need to reinvoke
Buildroot's make, for example to execute "make printvars".

However, so far post-build/image/fakeroot can't trivially run printvars
in a way that worked for both in-tree and out-of-tree builds. Indeed:

 * "make printvars" would work for in-tree builds, but not out of tree
   builds

 * "make -C ${O} printvars" would work for out-of-tree builds, but not
   in-tree builds

 * "make -C ${BR2_CONFIG%/*} printvars" works in both cases, but it is
   a bit cryptic, and two maintainers did not even immediately think of
   it

In order to solve this, this commit exposes $(CONFIG_DIR) to
post-build/image/fakeroot scripts, through the EXTRA_ENV variable.

The documentation is updated accordingly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr:
  - reference BR2_CONFIG as an exemple
  - slightly reword the commit log accordingly
  - move the doc for CONFIG_DIR next to that of BR2_CONFIG
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

docs/manual/customize-post-image.txt

@@ -28,7 +28,8 @@ different sets of arguments to each script.
 
 Again just like for the post-build scripts, the scripts have access to
 the environment variables +BR2_CONFIG+, +HOST_DIR+, +STAGING_DIR+,
-+TARGET_DIR+, +BUILD_DIR+, +BINARIES_DIR+ and +BASE_DIR+.
++TARGET_DIR+, +BUILD_DIR+, +BINARIES_DIR+, +CONFIG_DIR+ and
++BASE_DIR+.
 
 The post-image scripts will be executed as the user that executes
 Buildroot, which should normally _not_ be the root user. Therefore, any

docs/manual/customize-rootfs.txt

@@ -60,6 +60,9 @@ The post-build scripts are run with the main Buildroot tree as current
 In addition, you may also use these environment variables:
 
   - +BR2_CONFIG+: the path to the Buildroot .config file
+  - +CONFIG_DIR+: the directory containing the .config file, and
+    therefore the top-level Buildroot Makefile to use (which is
+    correct for both in-tree and out-of-tree builds)
   - +HOST_DIR+, +STAGING_DIR+, +TARGET_DIR+: see
     xref:generic-package-reference[]
   - +BUILD_DIR+: the directory where packages are extracted and built

package/Makefile.in

@@ -318,6 +318,7 @@ EXTRA_ENV = \
 	PATH=$(BR_PATH) \
 	BR2_DL_DIR=$(BR2_DL_DIR) \
 	BUILD_DIR=$(BUILD_DIR) \
+	CONFIG_DIR=$(CONFIG_DIR) \
 	O=$(CANONICAL_O)
 
 ################################################################################