linux: handle empty BR2_LINUX_KERNEL_IMAGE_TARGET_NAME situation

When BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM=y,
BR2_LINUX_KERNEL_IMAGE_TARGET_NAME should be set or the build will
fail. Instead of miserably failing the build, let's catch this earlier
and warn the user in a more informative way.

Related to:

  http://autobuild.buildroot.net/results/1d104a051c83bb31e98565369a2ec7badfa21eca/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit cb4f11a6964902990ce774ff7144b24a1ffd635c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/linux.mk

@@ -641,6 +641,10 @@ $(error No kernel device tree source specified, check your \
 	BR2_LINUX_KERNEL_INTREE_DTS_NAME / BR2_LINUX_KERNEL_CUSTOM_DTS_PATH settings)
 endif
 
+ifeq ($(BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM):$(call qstrip,$(BR2_LINUX_KERNEL_IMAGE_TARGET_NAME)),y:)
+$(error No image name specified in BR2_LINUX_KERNEL_IMAGE_TARGET_NAME despite BR2_LINUX_KERNEL_IMAGE_TARGET_CUSTOM=y)
+endif
+
 endif # BR_BUILDING
 
 $(eval $(kconfig-package))