This root Kconfig file "source"s (includes) the base gem5 src/Kconfig
file, and also any optional Kconfig files found in the base of EXTRAS
directories. These will be called out in the menuconfig interface and
config files with the name of the EXTRAS directory they came from, and a
blank section will be present either if the Kconfig didn't exist, or it
did exist but had no options in it.
Change-Id: I54060d613f0e0ab9372bed37a2fe5849bf5bbcdb