b8238df sysupgrade: support "backup" attribute
This update requires "sysupgrade" method callers to pass "backup"
attribute if $UPGRADE_BACKUP is used in the project.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 9785a9121d2d7a0a25bcd2924ee78fafada056da)