package/linux-headers: ensure tarball location is set

Validate that BR2_KERNEL_HEADERS_CUSTOM_TARBALL_LOCATION is set when
required.

Fixes:
http://autobuild.buildroot.net/results/f5e/f5e25cbb571c42f8a1728afa856a4fd54f4ad105

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

package/linux-headers/linux-headers.mk

@@ -35,6 +35,11 @@ endif # BR2_KERNEL_HEADERS_AS_KERNEL
 
 # Compute LINUX_HEADERS_SOURCE and LINUX_HEADERS_SITE from the configuration
 ifeq ($(LINUX_HEADERS_CUSTOM_TARBALL),y)
+ifeq ($(BR_BUILDING),y)
+ifeq ($(LINUX_HEADERS_CUSTOM_TARBALL_LOCATION),)
+$(error No kernel headers tarball location set, check your BR2_KERNEL_HEADERS_CUSTOM_TARBALL_LOCATION setting)
+endif
+endif
 LINUX_HEADERS_SOURCE = $(notdir $(LINUX_HEADERS_CUSTOM_TARBALL_LOCATION))
 LINUX_HEADERS_SITE = $(patsubst %/,%,$(dir $(LINUX_HEADERS_CUSTOM_TARBALL_LOCATION)))
 else ifeq ($(LINUX_HEADERS_CUSTOM_GIT),y)