net: lwm2m: Implement bootstrap discovery

Bootstrap discovery was not implemented properly in the LwM2M engine.

Although, there were some indications in the source code that it is
implemented, it was not done according to spec (and actually broken).

Given that Bootstrap Discovery procedure differs a lot from the regular
Device Management Discovery (different permissions, different
information returned), it's easier to implement it as a separate
function (`bootstrap_discovery()`) instead of making the existing
`do_discovery_op()` function even more complicated.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>