infra/pkg-kconfig: don't require a kconfig file for disabled packages

When a package is not enabled, we need not check that it provides
a .config/defconfig file.

So far, all our pkg-kconfig packages unconditionally set their
_KCONFIG_FILE, even to an empty value. But some packages (e.g.
at91bootstrap3) wanted to set it only when enabled, and broke.

So, only do the check when the package is enabled.

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/pkg-kconfig.mk

@@ -36,11 +36,6 @@ $(2)_KCONFIG_EDITORS ?= menuconfig
 $(2)_KCONFIG_OPTS ?=
 $(2)_KCONFIG_FIXUP_CMDS ?=
 
-# FOO_KCONFIG_FILE is required
-ifndef $(2)_KCONFIG_FILE
-$$(error Internal error: no value specified for $(2)_KCONFIG_FILE)
-endif
-
 # The config file could be in-tree, so before depending on it the package should
 # be extracted (and patched) first
 $$($(2)_KCONFIG_FILE): | $(1)-patch
@@ -72,6 +67,11 @@ $$($(2)_TARGET_CONFIGURE): $$($(2)_DIR)/.stamp_kconfig_fixup_done
 # already called above, so we can effectively use this variable.
 ifeq ($$($$($(2)_KCONFIG_VAR)),y)
 
+# FOO_KCONFIG_FILE is required
+ifndef $(2)_KCONFIG_FILE
+$$(error Internal error: no value specified for $(2)_KCONFIG_FILE)
+endif
+
 # Configuration editors (menuconfig, ...)
 $$(addprefix $(1)-,$$($(2)_KCONFIG_EDITORS)): $$($(2)_DIR)/.stamp_kconfig_fixup_done
 	$$($(2)_MAKE_ENV) $$(MAKE) -C $$($(2)_DIR) \