libipmi: Decouple user commands

This makes them an optional, separate provider like the sysfs cmds.

Tested:
    Ran on real hardware that included libusercmds.so in
    /usr/lib/host-ipmid and on a machine that did not include it. Ran
    ipmitool on the host to verify that the user commands were present
    when expected.

Change-Id: I07b3af5ae603c3e5f1ea41f0afc309329f4e185d
Signed-off-by: William A. Kennington III <wak@google.com>
6 files changed