linux: fix check of configuration file existence

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

linux/linux.mk

@@ -166,9 +166,11 @@ else ifeq ($(BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG),y)
 KERNEL_SOURCE_CONFIG = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE))
 endif
 
+ifeq ($(BR2_LINUX_KERNEL),y)
 ifeq ($(wildcard $(KERNEL_SOURCE_CONFIG)),)
 $(error Configuration file '$(KERNEL_SOURCE_CONFIG)' not found.)
 endif
+endif
 
 ifeq ($(call KCONFIG_GET_OPT,CONFIG_KERNEL_LZO,$(KERNEL_SOURCE_CONFIG)),y)
 LINUX_DEPENDENCIES += host-lzop