ar71xx: Create /var/lock directory in platform_do_upgrade_*

ar71xx: Create /var/lock directory in platform_do_upgrade_*

The sysupgrade_pre_upgrade hook was removed with 5e1b4c57ded7 ("base-files:
drop fwtool_pre_upgrade") while there were still scripts using it:

* target/linux/ar71xx/base-files/lib/upgrade/allnet.sh
* target/linux/ar71xx/base-files/lib/upgrade/openmesh.sh
* target/linux/ipq40xx/base-files/lib/upgrade/openmesh.sh

Not running the hooks can either prevent a successful upgrade or brick the
device because the fw_setenv program cannot be started correctly.

Instead of adding this hook again, the directory /var/lock for fw_setenv
can also just be created directly before fw_setenv is called.

Fixes: 5e1b4c57ded7 ("base-files: drop fwtool_pre_upgrade")
Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
(backported from c383e0c979fb95ef6de6782112aa86045fe4a6a5)