package/pkg-kconfig: fix *-diff-config under ppd

The 'linux-diff-config' target fails with the below error when
PER_PACKAGE_DIRECTORIES is enabled and the 'host-finalize' target hasn't
run yet.

  scripts/Kconfig.include:39: C compiler '.../buildroot/output/host/bin/arm-buildroot-linux-gnueabihf-gcc' not found

The 'PPD' variable isn't defined for this target, so 'BR_PATH' falls
back to the final host directory.

Signed-off-by: Brandon Maier <brandon.maier@collins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 641084bfb37856f79db86d9dbd6638ec70fc9681)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-kconfig.mk

@@ -301,6 +301,7 @@ endif
 # defconfig + fragments (if any) and the current configuration.
 # Note: it preserves the timestamp of the current configuration when moving it
 # around.
+$(1)-diff-config: PKG=$(2)
 $(1)-diff-config: $(1)-check-configuration-done
 	$$(Q)cp -a $$($(2)_DIR)/$$($(2)_KCONFIG_DOTCONFIG) $$($(2)_DIR)/.config.dc.bak
 	$$(call kconfig-package-merge-config,$(2),$$($(2)_DIR)/$$($(2)_KCONFIG_DOTCONFIG),\