toolchain-external-custom: error if BR2_TOOLCHAIN_EXTERNAL_URL is not set

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
[Arnout: add BR_BUILDING condition]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit d9a9d4df2b84350f7014b33b50af120c3915f4b5)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

toolchain/toolchain-external/toolchain-external-custom/toolchain-external-custom.mk

@@ -10,6 +10,11 @@ TOOLCHAIN_EXTERNAL_CUSTOM_SOURCE = $(notdir $(call qstrip,$(BR2_TOOLCHAIN_EXTERN
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_CUSTOM),y)
 # We can't check hashes for custom downloaded toolchains
 BR_NO_CHECK_HASH_FOR += $(TOOLCHAIN_EXTERNAL_SOURCE)
+ifeq ($(BR_BUILDING)$(BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD),yy)
+ifeq ($(TOOLCHAIN_EXTERNAL_CUSTOM_SOURCE),)
+$(error No external toolchain url set, check your BR2_TOOLCHAIN_EXTERNAL_URL setting)
+endif
+endif
 endif
 
 $(eval $(toolchain-external-package))