router: add option to include custom routes in RA

[DavidB137]

Introduces new configuration option ra_addroutes for adding custom route(s) in route advertisement (RA) messages when no default route is advertised.

This is useful when, for example, end devices are given only ULA IPv6 addresses (e.g. from fc00:0:100::/48), but one still needs inter subnet communication to e.g. fc00:0:200::/48. Currently, this wouldn't be possible without static route on each end device as the odhcpd server doesn't present itself as default router and no route information is given except for local ULA subnet (fc00:0:100::/48).

New configuration option ra_addroutes makes it possible to advertise arbitrary routes to the end devices and thus making correct routing possible even when no default route (or public IPv6 prefix) is available.

Option ra_addroutes exists under both odhcpd section and interfaces' sections. Latter sets up interface-specific routes. Routes configured under odhcpd are global and advertised for all interfaces (including those with interface-specific additional routes).

Config example:

config odhcpd 'odhcpd'
	...
	list ra_addroutes 'fc00:10::/32'
	list ra_addroutes 'fc00:20::/32'

config dhcp 'lan'
	...
	list ra_addroutes 'fd50:60:70::/48'

Fixes: #74 and possibly helps with #152

[systemcrash]

I think this is a good option to have.

@DavidB137 please fix the conflicts.

[DavidB137]

Sorry, I messed up quite a bit when resolving the conflicts. At first, I used spaces instead of tabs for indentation and then tried to fix it (unsuccessfully). So I instead synced my fork with upstream and reapplied the changes which should also make the history cleaner as there's no merge commit. Once again, sorry for the confusion.

@systemcrash Hopefully you can merge now.