odhcpd: reduce use of WITH_UBUS defines in code

odhcpd: reduce use of WITH_UBUS defines in code

By defining some dummy functions in the header file, we can avoid sprinkling
WITH_UBUS defines through the code, which helps readability and also ensures
that compilation errors (e.g. when a variable is renamed) is caught in builds
with and without ubus.

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/270/
Signed-off-by: Álvaro Fernández Rojas <[email protected]>