odhcp6c: add failure when -E option is used without ubus support enabled

odhcp6c: add failure when -E option is used without ubus support enabled

Problem: The -E option disables the state script and relies only on ubus
to emit events on state changes. But if odhcp6c is compiled without ubus
support (ENABLE_UBUS flag omitted), odhcp6c has no way to signal state
changes.

Solution: Return an error when -E flag is used and ubus support is
absent.

Signed-off-by: Nicolas BESNARD <[email protected]>
Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/106
Signed-off-by: Álvaro Fernández Rojas <[email protected]>