tools: patman: Handle tag sections without an 'END'

'Cover-letter', 'Series-notes' and 'Commit-notes' tags require an
'END' to be put at the end of its section. If we forget to put an
'END' in those sections, and these sections are followed by another
patman tag, patman generates incorrect patches. This adds codes to
handle such scenario.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>