Move makefile comment to separate line

One of the wonderful vagaries of GNU make is that adding a comment at
the end of an assignment will cause any trailing spaces before the
comment to end up in the variable.  This will cause problems if that
variable is later used as part of something that expects a single word,
such as ifdef.

Move the comment to the line above the assignment.

Change-Id: Idfa6c28c4941cd565e96b229c2eec9bb915d37ed