lib, tools: silence harmless warnings in the northbound tools

lib, tools: silence harmless warnings in the northbound tools

Our two northbound tools don't have embedded YANG modules like the
other FRR binaries. As such, ly_ctx_set_module_imp_clb() shouldn't be
called when the YANG subsystem it being initialized by a northbound
tool. To make that possible, add a new "embedded_modules" parameter
to the yang_init() function to control whether libyang should look
for embedded modules or not.

With this fix, "gen_northbound_callbacks" and "gen_yang_deviations"
won't emit "YANG model X not embedded, trying external file"
warnings anymore.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>