pkg-kconfig: declare phony targets as such

Just like the previous commit did for the pkg-generic infrastructure,
this commit improves the pkg-kconfig infrastructure to declare its
custom <pkg>-<something> targets as PHONY.

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

package/pkg-kconfig.mk

@@ -100,6 +100,12 @@ $(1)-update-defconfig: $(1)-savedefconfig
 
 endif # package enabled
 
+.PHONY: \
+	$(1)-update-config \
+	$(1)-update-defconfig \
+	$(1)-savedefconfig \
+	$$(addprefix $(1)-,$$($(2)_KCONFIG_EDITORS))
+
 endef # inner-kconfig-package
 
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