package/mender: install additional inventory scripts

There are two additional inventory scripts provided in
the Mender client repository, let's install them.

- mender-inventory-os
    - will push content of /etc/os-release
- mender-inventory-rootfs-type
    - will push filesystem type of rootfs

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>