classes/populate_sdk_ext: add a better config extension mechanism
The sdk_extraconf() method of setting the configuration was awkward
since you needed to set it in a class and then inherit that class since
function definitions aren't allowed in conf files. It seemed to me the
a neater way to do this was to read the extra lines from an additional
conf file sdk-extra.conf (which can be located in a conf/ directory
anywhere along BBPATH as with other configuration files).
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
