Implement HMAC-SHA256 in xCAT IPMI

[jjohnson42]

This will be compatible with some current implementations that do not have SHA-1. HMAC-SHA1 may be still secure, but SHA1 in any context looks bad even if it isn't.

[cxhong]

@jjohnson42 , how did u run unit test for this?

[MBlaschek]

Not sure why it did not make into the 2.16.2 release? I use 2.16.5 and it is not there. I manually changed your additions! It works. Thanks.