script to install yang models

pkg-config libsysrepo --variable=SR_REPOSITORY_LOC returns the sysrepo repository directory so we can:

• use it in configure to set a autoconf variable to it

• update the script in tools to do the same thing

• provide a script in the models directory to install it. As it should be run once it is enough.

• update the documentation (i.e. netconf.xml)

• port this to kea-yang