[yocto] [meta-mono] Mono updated to 3.12.0

[Alex J Lennon]

I've updated support for Mono build in meta-mono master to 3.12.0 (the
current release).

I've performed basic build testing on an qemux86 build of
core-image-mono, based on core-image-sato, running a "Hello World"
console application and a "Hello World" Windows Forms application.

There has been feedback that issues with armhf support are addressed in
3.10.0/3.12.0 but I have not yet had time to investigate.

ref: https://bugzilla.xamarin.com/show_bug.cgi?id=20239

All feedback on testing and/or issues appreciated.

Regards,

Alex