libsa: Move hash functions up a level

This should have no functional change. Move compiling the sha256, sha512
and md5 hash functions up into libsa to allow them to be used elsewhere
in the boot loader when geli isn't configured. Since libsa is a .a, these
won't wind up in any boot loader that doesn't reference them, so should
be a nop.

Sponsored by: Netflix

(cherry picked from commit fd577b59b2d78a32f40fdc3847666728d8a897db)