patman: fix series-notes handling for buildman

A patman series with a 'Series-notes' section causes
buildman to crash with:

    self.series.notes += self.section
    TypeError: cannot concatenate 'str' and 'list' objects

Fix by initializing series.notes as a one-element array
rather than a scalar.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>