Accept Change-Id even if footer contains not well-formed entries

Accept Change-Id even if footer contains not well-formed entries

Instead of only looking for a Change-Id in the last section if it
consists only of well-formed "key: value" lines replace the last
occurrence of a valid Change-Id line in the last section. Some tools
require footer lines e.g. without a colon.

Gerrit doesn't accept Change-Id lines in the footer if the Change-Id
line doesn't start at the beginning of the line.

Bug: 400818
Change-Id: Icce54872adc8c566994beea848448a2f7ca87085
Signed-off-by: Stefan Lay <stefan.lay@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>