secure: hook up libecc as libpkgecc

libecc is not intended to be general use, other applications should
really be using openssl. pkg(7) uses libecc to align with the pkg(8)
project and its goals. This will be used in the upcoming support for
ECC in pkg(7).

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D48117