Remove iputils-ping retrocompatibility with OpenWrt 19.07

[ilario]

Fix #794

The ping6retrocompat.mk file was used for compiling with OpenWrt releases having split iputils-ping and iputils-ping6 or single package iputils-ping.

For some reason, it did not work for me, and it is not needed anymore as we are not supporting OpenWrt 19.07 on the master branch (the OpenWrt_19.07_compatible branch has been created for that).

[G10h4ck]

Removing $(PING6_SYMLINK) dependency will cause ping4 and ping6 not being provided by iputils-ping anymore so shared-state will be broken because it uses ping6 you should also change the occurrences of ping6 into ping -6 at least into shared-state-get_candidates_neigh

[ilario]

Removing $(PING6_SYMLINK) dependency will cause ping4 and ping6 not being provided by iputils-ping anymore so shared-state will be broken because it uses ping6 you should also change the occurrences of ping6 into ping -6 at least into shared-state-get_candidates_neigh

Right, will do. Thanks!

[ilario]

you should also change the occurrences of ping6 into ping -6

Isn't it enough to use ping instead of ping -6?
I did not test yet if they are equivalent, do you have insight on this?

[ilario]

@selankon please check if the modification of get_loss can have any impact on lime-app

I added some commits where I replaced all the occurrences of ping6 with ping.
I did not include the -6 option to ping for decreasing the complexity of the code (you will see that I got rid of some code that was checking if an IP is v4 or v6).
But it would be easy to just revert the commits and replace ping6 with ping -6, if needed.

[G10h4ck]

should be fine, please test with a few of routers if shared-state keeps discovering neighbors and then we can merge this