meta/conf/templates/default/conf-summary.txt: add a template summary

meta/conf/templates/default/conf-summary.txt: add a template summary

Existing conf-notes.txt is not adequate in situations where the user
needs to select a config template from several, or get an overview of them and
is interested only in a brief explanation of what the template is for,
but not extended multi-paragraph instructions for how to use it.

Such summaries can be provided via conf-summary.txt and
this patch adds such a summary for the oe-core default template.

Updates to the tools and a tool for selecting templates which
will make specific use of the summary will follow.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>