base-files: move stage2 upgrade to separated file

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 f7edd94a65)

package/base-files/files/sbin/sysupgrade

@@ -363,7 +363,7 @@ fi
 install_bin /sbin/upgraded
 v "Commencing upgrade. Closing all shell sessions."
 
-COMMAND='. /lib/functions.sh; include /lib/upgrade; do_upgrade_stage2'
+COMMAND='/lib/upgrade/do_stage2'
 
 if [ -n "$FAILSAFE" ]; then
 	printf '%s\x00%s\x00%s' "$RAM_ROOT" "$IMAGE" "$COMMAND" >/tmp/sysupgrade