Re-add sbsigntool for SecureBoot systems that was removed in r501106.
All that was needed to compile on FreeBSD 11 was to add in a
BUILD_DEPEND for bfd.h from devel/libbfd.
For FreeBSD 12 this needed a move to the OpenSSL 1.1 API's.
was used as a crib sheet. I'm not adding in shims or ifdefs so that
this still builds for FreeBSD 11 (unless there is later demand for it).