core/show-info: report build_dir relative to CONFIG_DIR

Currently, the build_dir field is reported relative to $(BASE_DIR), to
avoid leaking local paths.

However, BASE_DIR is not a directory that is very convenient: for
in-tree builds, it is $(CONFIG_DIR)/output/, while for out-of-tree
builds, it is $(CONFIG_DIR). This difference is purely an idiosyncracy
of how out-of-tree builds have been implemented in Buildroot, and is
not under the control of the user.

What the user is in control of, however, is where the .config file is
located. This, really, is the directory we should base relative paths
on.

Reported-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/pkg-utils.mk

@@ -113,7 +113,7 @@ define _json-info-pkg
 		"virtual": false$(comma)
 		$(call _json-info-pkg-details,$(1)) \
 	)
-	"build_dir": "$(patsubst $(BASE_DIR)/%,%,$($(1)_BUILDDIR))",
+	"build_dir": "$(patsubst $(CONFIG_DIR)/%,%,$($(1)_BUILDDIR))",
 	$(if $(filter target,$($(1)_TYPE)), \
 		"install_target": $(call yesno-to-bool,$($(1)_INSTALL_TARGET))$(comma) \
 		"install_staging": $(call yesno-to-bool,$($(1)_INSTALL_STAGING))$(comma) \