kernel-yocto.bbclass: Reset to exiting on non-zero return code at end of task

Several tasks deactivate exiting on non-zero return codes via set +e because
they run subcommands that have legitimate non-zero return codes. However when
appending to those tasks, this behavior is not expected and can lead to builds
silently proceeding in case of an error. Therefore reset the default behavior
at the end of the respective tasks via set -e.

Signed-off-by: Stefan Wiehler <stefan.wiehler@nokia.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 83a6f28d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>