linux/linux.mk: ensure custom kernel version is set

Validate that BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE is set when
required.

Fixes:
http://autobuild.buildroot.net/results/117/117c08e414776202446a9fdb96d084aa3c16af48

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

linux/linux.mk

@@ -566,6 +566,12 @@ endif
 
 ifeq ($(BR_BUILDING),y)
 
+ifeq ($(BR2_LINUX_KERNEL_CUSTOM_VERSION),y)
+ifeq ($(LINUX_VERSION),)
+$(error No custom kernel version set. Check your BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE setting)
+endif
+endif
+
 ifeq ($(BR2_LINUX_KERNEL_USE_DEFCONFIG),y)
 # We must use the user-supplied kconfig value, because
 # LINUX_KCONFIG_DEFCONFIG will at least contain the