combo-layer-hook-default.sh: handle patches without Signed-off-by
Inserting the "From rev" comment depended on having at least one
Signed-off-by line in the patch header. Some old repository commits in
openembedded-core and bitbake do not have those.
When inporting those, just insert at the end of the patch
header. While doing so, ensure that there's exactly one blank line
since the last non-blank line.
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
