tools: env: use /run to store lockfile

tools: env: use /run to store lockfile

According this issue https://github.com/ppp-project/ppp/issues/339.
Eventually, the dt-utils changed lock directory to fix missing /var/lock
directory error then make dt-utils can run normally.

We also have a similar issue with these two utilities fw_printenv and
fw_setenv will failed when the directory /var/lock is non-existent.

We have a custom linux distribution built with yocto (OpenBMC) that
use systemd and it deprecated the /var/lock directory.
More discussion in systemd/systemd#15668.

Thus, we sync with community's solution for uboot/tools/env utilities:
The current location /var/lock is considered legacy (at least by systemd).
Just use /run to store the lockfile and append the usual .lock suffix.

Tested:
Verified /run/lock is now present and fw_printenv can work in OpenBMC.

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>