config: use multi-stage parsing of uci sections

config: use multi-stage parsing of uci sections

When loading uci sections from config file, in one pass do not
mix sections from different types. First load odhcpd global
settings, then all interface sections and finally static leases.
It ensures that section order in file can't affect what information
is already parsed. For example static lease section may need
information about all interfaces, to decide whether ip address
belongs to any of currently defined interfaces/address pools.

Signed-off-by: Dainis Jonitis <dainis.jonitis@ubnt.com>