Reminder: update development instructions to OpenWrt 19.07-specific packages

[ilario]

Follow-up from #103

When the build instructions will suggest to compile based on OpenWrt 19.07, we have to remember to suggest the users to select batctl-default instead of batctl. Additionally, the fft-eval package is finally available and should be included in the suggested packages list.

[ilario]

Closing this:
suggesting to include fft-eval would be confusing for the users compiling the next release on top of OpenWrt 18.06 where it is not present.
And we do not need to depend upon batctl: libremesh/lime-packages#673 (comment)

[ilario]

Maybe now fft-eval should be added to the compilation instructions?

[ilario]

fft-eval is used only in ubus-lime-openairview but this is not used in lime-app (only in the non-recommended LuCI-based interface). So there is no easy way for the users to use it (and anyway it is available only for ath9k). Unless anyone wants to add it to lime-app, I think we should not recommend selecting it.