libyang from network project

For 16.0 run the following as root:

zypper addrepo https://download.opensuse.org/repositories/network/16.0/network.repo
zypper refresh
zypper install libyang

For 15.7 run the following as root:

zypper addrepo https://download.opensuse.org/repositories/network/15.7/network.repo
zypper refresh
zypper install libyang

For 15.6 run the following as root:

zypper addrepo https://download.opensuse.org/repositories/network/15.6/network.repo
zypper refresh
zypper install libyang