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>