doc: update build instructions for freebsd on how to obtain libyang
Unfortunately the first version of the FreeBSD libyang port contained
a bug in which the libyang pkginfo file wasn't being installed
correctly in the system, and this prevented the FRR build system from
detecting the library. This bug was already fixed months ago but some
FreeBSD package repositories still have the old bugged version of the
port. This means we can't suggest people to install libyang using
"pkg install" since this causes problems for most people. In this
case, suggest FreeBSD users to build and install libyang manually
as we suggest for other BSD platforms.
This commit should be reverted once all FreeBSD package repositories
are updated with the new version of the libyang port.