build: Document outputexports

In addition to providing documentaton, we also modify the static_lib
example to use outputexports instead of the double-include
Makefile.toolchain trick. For this to work well we need to fix a tiny
bug in the dependency handling in Makefile.inc.

Change-Id: I6571bc681bc34155f37cff1eccc2ea12ed52ef07
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>