at91bootstrap3: make sure AT91BOOTSTRAP3_KCONFIG_FILE is always defined

For some (silly?) reason, the kconfig-package infra always wants
<pkg>_KCONFIG_FILE to be defined. So let's do like barebox does: use a
temporary AT91BOOTSTRAP3_SOURCE_CONFIG variable, and assign it to
AT91BOOTSTRAP3_KCONFIG_FILE. This way, the latter is always defined,
even if to the empty value.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/at91bootstrap3/at91bootstrap3.mk

@@ -32,10 +32,12 @@ define AT91BOOTSTRAP3_INSTALL_IMAGES_CMDS
 endef
 
 ifeq ($(BR2_TARGET_AT91BOOTSTRAP3_USE_DEFCONFIG),y)
-AT91BOOTSTRAP3_KCONFIG_FILE = $(AT91BOOTSTRAP3_DIR)/board/*/$(call qstrip,$(BR2_TARGET_AT91BOOTSTRAP3_DEFCONFIG))_defconfig
+AT91BOOTSTRAP3_SOURCE_CONFIG = $(AT91BOOTSTRAP3_DIR)/board/*/$(call qstrip,$(BR2_TARGET_AT91BOOTSTRAP3_DEFCONFIG))_defconfig
 else ifeq ($(BR2_TARGET_AT91BOOTSTRAP3_USE_CUSTOM_CONFIG),y)
-AT91BOOTSTRAP3_KCONFIG_FILE = $(call qstrip,$(BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_CONFIG_FILE))
+AT91BOOTSTRAP3_SOURCE_CONFIG = $(call qstrip,$(BR2_TARGET_AT91BOOTSTRAP3_CUSTOM_CONFIG_FILE))
 endif
+
+AT91BOOTSTRAP3_KCONFIG_FILE = $(AT91BOOTSTRAP3_SOURCE_CONFIG)
 AT91BOOTSTRAP3_KCONFIG_EDITORS = menuconfig xconfig gconfig
 AT91BOOTSTRAP3_KCONFIG_OPTS = $(AT91BOOTSTRAP3_MAKE_OPTS)
 $(eval $(kconfig-package))