[yocto] [meta-mono] Mono updated to 3.8.0

[Alex J Lennon]

I've updated support for Mono build in meta-mono master from 3.4.0 to
3.8.0 (the current release).

I've performed basic build testing on an x64 host targetting an i.MX6
platform and succesfully run up a commercial application which makes use
of console / X.

If anybody has the time and inclination to perform build and smoke
testing all feedback would be appreciated and incorporated into the README.

Note that there is a still an outstanding issue with Mono not working
correctly with ARM hardfp targets.

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

I have been unable to determine why this is to date. If anybody is
interested in working with me to track down the issue please contact me
directly.

Cheers,

Alex