env: tool: add command line option to input lockfile path

The default lockname is set to /var/lock. This limits the
usage of this application where OS uses different lockfile
location parameter.
For example, In case of android, the default lock
path location is /data.
Hence by providing the command line option to input lockfile
path will be useful to reuse the tool across multiple
operating system.

usage: ./fw_printenv -l <lockfile path>

Signed-off-by: Ravi Babu <ravibabu@ti.com>