libefivar: Replace BSD License with BSD+Patent License

https://bugzilla.tianocore.org/show_bug.cgi?id=1373

Replace BSD 2-Clause License with BSD+Patent License. This change is
based on the following emails:

https://lists.01.org/pipermail/edk2-devel/2019-February/036260.html
https://lists.01.org/pipermail/edk2-devel/2018-October/030385.html

RFCs with detailed process for the license change:

V3: https://lists.01.org/pipermail/edk2-devel/2019-March/038116.html
V2: https://lists.01.org/pipermail/edk2-devel/2019-March/037669.html
V1: https://lists.01.org/pipermail/edk2-devel/2019-March/037500.html

Note that the link above no longer exists. The commit message was kept
verbatim. An archive of the bug report can be found at:
https://web.archive.org/web/20240714192319/https://bugzilla.tianocore.org/show_bug.cgi?id=1373

Obtained from: https://github.com/tianocore/edk2/commit/9344f0921518309295da89c221d10cbead8531aa

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1894