linux: explain why we need host-{flex, bison}

The commit that added the dependency on host-{bison,flex} did
so because the pre-generated kconfig parser source files were
removed from the kernel tree, in linux-4.16.

But then, in linux-4.17, the pre-generated dtc parser source
files were in turn removed as well.

So, document the two reasons why they are needed, so we don't
accidentally remove them when we (soon) introduce the kconfig
dependencies.

(Also fix the first assignment to LINUX_DEPENDENCIES to be a
simple assignement, not an append-assignment.)

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>