[makefiles] Add __simplify, which replaces a recursively-expanded variable with a simply-expanded variable, so we don't recalculate too many things unnecessarily.

From: Dustin L. Howett <dustin@howett.net>

git-svn-id: http://svn.howett.net/svn/theos/trunk@601 4410221e-0ddf-4ce3-99c0-2db6c0dbc727