Start of 4.3 migration guide docs

[Richard Purdie <richard.purdie@linuxfoundation.org>]

As mentioned on the weekly tech call, we need to get better about
documenting changes for the migration guide and release notes as we go
along. Whilst we're not far into 4.3, I realise I made or merged
changes already which will need notes. Since we don't have a guide yet,
let me note them here on the basis Michael will inject them into a new
doc.


* The '-crosssdk' suffix and any MLPREFIX were removed from virtual/XXX
provider/dependencies where a PREFIX was used as well as we don't need
both and it made automated dependency rewriting unnecessarily complex.
In general this only affects internal toolchain dependencies so isn't
end user visible.

* runtime testing of ptest now fails if no test results are returned by
any given ptest.

* the bitbake cooker log now contains notes when the caches are
invalidated which is useful for memory resident bitbake debugging.

* glide was dropped as "go mod" has become the standard


Cheers,

Richard