This explicitly tells procd what backup file should be used during
sysupgrade (if any). It's much more generic this way compared to the
magic /tmp/sysupgrade.tgz file that had to be created before a call.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit c5223b26a40ae61fc7750bf865464048af328ab1)
Rafał Miłecki