core/pkg-kconfig: don't enforce check for config file when not building

Currently, this is triggering the error message:
    make randconfig
    make source

Only limit the check to enforce a config file being set to when we are
actually building, like is done in the various kconfig-using packages.

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

package/pkg-kconfig.mk

@@ -90,9 +90,11 @@ $$($(2)_TARGET_CONFIGURE): $$($(2)_DIR)/.stamp_kconfig_fixup_done
 ifeq ($$($$($(2)_KCONFIG_VAR)),y)
 
 # FOO_KCONFIG_FILE is required
+ifeq ($$(BR_BUILDING),y)
 ifeq ($$($(2)_KCONFIG_FILE),)
 $$(error Internal error: no value specified for $(2)_KCONFIG_FILE)
 endif
+endif
 
 # Configuration editors (menuconfig, ...)
 #