sim: Add an option to load additional kernel objects

sim: Add an option to load additional kernel objects

There are cases where it is desirable to load a kernel and a set of
additional objects. This can, for example, be useful for testing where
the bootstrap code can be loaded from one object (the kernel) and the
test proper from another.

This changeset adds this functionality by adding a kernel_extras
vector parameter to the System class. Object files in this vector are
loaded in order after the kernel when running in full system mode.

Change-Id: I06f57c6a65a17b02eb4267bed0aa829f21bcfa3b
Reviewed-on: https://gem5-review.googlesource.com/5703
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>