recipetool: create: support extracting SUMMARY and HOMEPAGE

recipetool: create: support extracting SUMMARY and HOMEPAGE

Allow plugins to set any variable value through the extravalues dict,
and use this to support extracting SUMMARY and HOMEPAGE values from spec
files included with the source; additionally translate "License:" to a
comment next to the LICENSE field (we have our own logic for setting
LICENSE, but it will often be useful to see what the spec file says if
one is present).

Also use the same mechanism for setting the same variables for node.js
modules; this was already supported but wasn't inserting the settings in
the appropriate place in the file which this will now do.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>