config: Add new MemConfig options to DRAM sweep script

Update script to match current MemConfig options with
external_memory_system option set to 0.

configs/dram/sweep.py

@@ -96,6 +96,7 @@ system.mem_ranges = [mem_range]
 # force a single channel to match the assumptions in the DRAM traffic
 # generator
 options.mem_channels = 1
+options.external_memory_system = 0
 MemConfig.config_mem(options, system)
 
 # the following assumes that we are using the native DRAM