arm-autonomy: Move machine override soft assignments to inc files

Moved assignments of the form:
     MYVAR_$MACHINE ?= "val"
to a machine specific .inc file, as the above will still set the
value when the base variable has been set in local.conf, e.g:
     MYVAR = "local_val"
despite the weak assignment.

Change-Id: I3ba77453be785ec92ab45255cbd34d7d77bbcf37
Issue-Id: SCM-1524
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>