linux: host-{flex, bison} only needed for DTS

host-{flex,bison} are only needed to generate the dtc parser, so we
don't need them if the kernel does not have support for device tree.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>