linux: check that a repo and version is defined when using a custom repo

If not set the system will use an empty string which will result in
download errors for 'linux-.tar.gz' packages.

This patch makes it obvious to the user that the variable needs to be
set.

Signed-off-by: Christian Kellermann <christian.kellermann@solectrix.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

linux/linux.mk

@@ -428,6 +428,17 @@ LINUX_PRE_PATCH_HOOKS += $(foreach ext,$(LINUX_EXTENSIONS),\
 		$(call UPPERCASE,$(ext))_PREPARE_KERNEL))
 
 # Checks to give errors that the user can understand
+
+# When a custom repository has been set, check for the repository version
+ifeq ($(BR2_LINUX_KERNEL_CUSTOM_SVN)$(BR2_LINUX_KERNEL_CUSTOM_GIT)$(BR2_LINUX_KERNEL_CUSTOM_HG),y)
+ifeq ($(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION)),)
+$(error No custom repository version set. Check your BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION setting)
+endif
+ifeq ($(call qstrip,$(BR2_LINUX_KERNEL_CUSTOM_REPO_URL)),)
+$(error No custom repo URL set. Check your BR2_LINUX_KERNEL_CUSTOM_REPO_URL setting)
+endif
+endif
+
 ifeq ($(BR_BUILDING),y)
 
 ifeq ($(BR2_LINUX_KERNEL_USE_DEFCONFIG),y)