linux: check the configuration file exists

... and abort early, before we even use it.

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

linux/linux.mk

@@ -166,6 +166,10 @@ else ifeq ($(BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG),y)
 KERNEL_SOURCE_CONFIG = $(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE))
 endif
 
+ifeq ($(wildcard $(KERNEL_SOURCE_CONFIG)),)
+$(error Configuration file '$(KERNEL_SOURCE_CONFIG)' not found.)
+endif
+
 ifeq ($(call KCONFIG_GET_OPT,CONFIG_KERNEL_LZO,$(KERNEL_SOURCE_CONFIG)),y)
 LINUX_DEPENDENCIES += host-lzop
 endif