Re-add sbsigntool for SecureBoot systems that was removed in r501106.

[obrien]

Summary

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.
https://git.kernel.org/pub/scm/linux/kernel/git/jejb/sbsigntools.git/commit/?id=79d0c44afbc7da0adefe1250a17a49e6eb90c9bd
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).

Test Plan

Installed on FreeBSD 12.2 and validated against Juniper Network's SecureBoot loader.

[obrien]

Diff #2

[obrien]

More accurately depend on libbfd port.