license: switch to Apache

We have always preferred Apache license for OpenBMC, but when this
repository was first created someone checked in a GPL "COPYING" file
and it was never updated.  I looked at all the Authors in this
repository and all of them work for a company who has signed a CCLA.
Therefore, I don't think there should be any issue with aligning the
copyright license to Apache going forward on this repository.

Signed-off-by: Patrick Williams <[email protected]>
Change-Id: I6f4bfc108ea3e817fbc66cc4d83ce51fe560305a

Author: williamspatrick