config: Embed Device Tree generation in fs.py config

config: Embed Device Tree generation in fs.py config

Equips the fs.py config routine with an extra commandline option
--generate-dtb that will generate a dtb file automatically before
running the simulation. Only works with ARM systems and gives a warning
if the simulated system is not of --machine-type VExpress_GEM5_V1.

Change-Id: I7766e5459fd9bec2245de83cef103091ebaf7229
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5968
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>