opkg: fix for MEIP-275

Change location of lock file from /var/lib/opkg/lock to
/var/volatile/opkg/lock so that it can work with ro rootfs.

Signed-off-by: Sanjeev Chugh <sanjeev_chugh@mentor.com>
Merge pull request #33 from sanjeevc/MEIP-275

opkg: fix for MEIP-275