do_upgrade_stage2() isn't really any common code. It isn't used anywhere
except for /sbin/sysupgrade that passes it to the stage2.
Moving its code to separated file also simplifies COMMAND variable.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit f7edd94a6588aa6a3e2e7a72410a34392a9fbe78)