router: improve RA logging

router: improve RA logging

We only set the RA lifetime to what is configured in UCI when there is a
default route and valid prefix. In any other case, we set it to 0. This
leads to confusion where people believe ra_lifetime is completely
ignored. In case there is a default route, but no valid prefix, a debug
message explains this, but if there is no default route, we silently
override ra_lifetime.

Add a debug message for the latter case, and explicitly mention
overriding ra_lifetime in both cases.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Hans Dedecker <dedeckeh@gmail.com>