base-files: Reintroduce sysupgrade_pre_upgrade hook

base-files: Reintroduce sysupgrade_pre_upgrade hook

The sysupgrade_pre_upgrade hook was removed with 6a27c2f4b1a4 ("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.

Fixes: 6a27c2f4b1a4 ("base-files: drop fwtool_pre_upgrade")
Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>