linux: kconfig needs the toolchain

Starting with linux-4.18, the kconfig from the kernel can call
to the compiler to test its capabilities; see:

    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/scripts/Kconfig.include

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Jan Kundrát <jan.kundrat@cesnet.cz>
Tested-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Reviewed-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

linux/linux.mk

@@ -71,6 +71,11 @@ LINUX_KCONFIG_DEPENDENCIES = \
 	$(BR2_BISON_HOST_DEPENDENCY) \
 	$(BR2_FLEX_HOST_DEPENDENCY)
 
+# Starting with 4.18, the kconfig in the kernel calls the
+# cross-compiler to check its capabilities. So we need the
+# toolchain before we can call the configurators.
+LINUX_KCONFIG_DEPENDENCIES += toolchain
+
 # host tools needed for kernel compression
 ifeq ($(BR2_LINUX_KERNEL_LZ4),y)
 LINUX_DEPENDENCIES += host-lz4