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

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>