ar71xx: Fix sysupgrades for Allnet and OpenMesh devices

Both Allnet and OpenMesh sysupgrade uses fw_setenv from uboot-envtools to
change different settings in the uboot environment. uboot-envtools version
2013.01 introduced a filesystem lock "/var/lock/fw_printenv.lock" to guarantee
mutually exclusive access to the uboot environment. But the path /var doesn't
exist on the sysupgrade ramfs.

An upgrade on these devices fails since r36033 ("[package] uboot-envtools:
upgrade to version 2013.01.01") with following messages:

 Error opening lock file /var/lock/fw_printenv.lock
 failed to update U-Boot environment

Creating the "/var/lock" path before running fw_setenv is therefore a
requirement unless the locking functionality in fw_setenv is removed or
replaced with optional locking.

Signed-off-by: Sven Eckelmann <sven@open-mesh.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 38483