Add sys restart hook; change mgos_system_restart

Drop the argument to mgos_system_restart, it wasn't used anyway.
Move public parts of mgos_hal into mgos_system.h, functions left in mgos_hal are strictly internal.

PUBLISHED_FROM=ec1c061a8e45e22e3fc1c1509a9dc99b2b522358

Author: Deomid Ryabkov