Add config option for default discard post-measure

[aricooperdavis]

Hi Phil,

Further to my email, here are those changes as a pull-request.

I'll have a go at compiling it myself too.

Many thanks,
Ari

[aricooperdavis]

Hey Phil,
I'm happy for this PR to be merged if you are? It's a non-breaking change (only adds a new option to the menu).
I've only been able to test it on Matt's bluetooth SAP5, but given that I've not touched any BT code I can't imagine it'll have any problems on non-BT SAPs.
All the best,
Ari

[aricooperdavis]

For anyone following along, here's an early version of the firmware reflecting this change. Note that the menu entry is too long so is cut off the screen, but it still works fine.