make: flatten object file directories

Rather than building an object file directory heirarchy matching the
source tree's layout, rename the object files so that the object
file name contains the path in the source file tree. The intent here
is to allow two files in different parts of the source tree to have
the same name and still not collide when put into an ar archive.

Change-Id: Id627737dc95ffc65b738501215f34a995148c5a2