busybox: sysntpd: option to bind server to iface

busybox: sysntpd: option to bind server to iface

NTPD in busybox has option -I to bind server to IFACE.
However, capabilities of the busybox are limited, the -I option cannot be
repeated and only one interface can be effectively specified in it.
This option is currently not configurable via UCI.
The patch adds an interface option to the system config, ntp section.
Also sort options for uci_load_validate alphabetically.

Signed-off-by: Alexey Dobrovolsky <dobrovolskiy.alexey@gmail.com>
(cherry picked from commit e12fcf0fe5597467f7cc21144e5f4da60500ebd2)