MFC: r287579

MFC: r287579

Implement pubkey support for the bootstrap

Note that to not interfer with finger print it expects a signature on pkg itself
which is named pkg.txz.pubkeysign

To genrate it:
echo -n "$(sha256 -q pkg.txz)" | openssl dgst -sha256 -sign /thekey \
    -binary -out ./pkg.txz.pubkeysig

Note the "echo -n" which prevent signing the '\n' one would get otherwise

PR:		202622

git-svn-id: svn://svn.freebsd.org/base/stable/10@287809 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f