doc: Restore documentation for 'make outputexports'

Commit 7cb8a16c ("doc: restructure application primer") removed
the section documenting the build system support for third party
libraries. Restore this section making a few editorial changes to
ensure the text sits well in its surroundings.

Jira: ZEP-1733
Change-Id: Ie62b956732f36fac70b392eeee880acebaef6cf9
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>