buildroot: Add Ghn init scripts

Fully set up and configure G.hn chip on GFLT400:
 - S45ghn brings up interface and starts ghn-periodic-stats
 - ghn-periodic-stats writes stats to file every 60s for cwmp
     see go/fibercl/83380
 - read-ghn-local is wrapper around read-ghn-node
     see go/fibercl/83450

Change-Id: I3cee89075f336e20823d990ee5cdd732dae95a8e
3 files changed