Display a warning when a module is already defined and proceed to define the new module.  This will allow builds to proceed when receiving an error such as build/core/base_rules.mk:130: *** device/samsung/d2-common/rootdir: MODULE.TARGET.ETC.init.target.rc already defined by device/htc/ville/rootdir.  Stop.

Change-Id: I97191023ff8240642a4726e4893d715062a03c63