This var has been replaced by the $UPGRADE_OPT_UPGRADE_OPT_SAVE_CONFIG
Fixes: f25d164aca80 ("base-files: pass "save_config" option to the "sysupgrade" method")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 5797fe84a3b508483f7d82e177157c1bf2f342d9)